Support gradle.properties version file (#5)

The `gradle.properties` file allows for a default version while still
allowing for the version to be overridden on the command line. This is a
perfect scenario for new-release-version

For example:
Set the property only in the `gradle.properties` file (i.e. remove it
from build.gradle). Also make sure the options come before the command
(as mentioned above).

gradle.properties contents:

```
version=1.0.12
```

Version can then be overridden on the command line with:

```
gradle -Pversion=1.0.13 publish
```

Based on the StackOverflow answer
https://stackoverflow.com/a/40239092/620196

examples/java/gradle.properties/build.gradle

@@ -0,0 +1,29 @@
+/*
+ * This file was generated by the Gradle 'init' task.
+ *
+ * This generated file contains a sample Java Library project to get you started.
+ * For more details take a look at the Java Libraries chapter in the Gradle
+ * User Manual available at https://docs.gradle.org/6.1.1/userguide/java_library_plugin.html
+ */
+
+plugins {
+    // Apply the java-library plugin to add support for Java Library
+    id 'java-library'
+}
+
+repositories {
+    // Use jcenter for resolving dependencies.
+    // You can declare any Maven/Ivy/file repository here.
+    jcenter()
+}
+
+dependencies {
+    // This dependency is exported to consumers, that is to say found on their compile classpath.
+    api 'org.apache.commons:commons-math3:3.6.1'
+
+    // This dependency is used internally, and not exposed to consumers on their own compile classpath.
+    implementation 'com.google.guava:guava:28.1-jre'
+
+    // Use JUnit test framework
+    testImplementation 'junit:junit:4.12'
+}

examples/java/gradle.properties/gradle.properties

@@ -0,0 +1 @@
+version=1.3.2

newrelver.go

@@ -21,9 +21,9 @@ const VersionNumberRegex = `[\.\d]+(-\w+)?`
 //
 // The %s is replaced with VersionNumberRegex.
 const (
-	VersionsGradleRegexf = `(?m)project\.version\s*=\s*['"](%s)['"]$`
+	ConfigRegexf         = `(?m)^version\s*=\s*(%s)$`
 	BuildGradleRegexf    = `(?m)^version\s*=\s*['"](%s)['"]$`
-	SetupCfgRegexf       = `(?m)^version\s*=\s*(%s)$`
+	VersionsGradleRegexf = `(?m)project\.version\s*=\s*['"](%s)['"]$`
 	SetupPyRegexf        = `(?ms)setup\(.*\s+version\s*=\s*['"](%s)['"].*\)$`
 	CMakeListsTxtRegexf  = `(?ms)^project\s*\(.*\s+VERSION\s+(%s).*\)$`
 	MakefileRegexf       = `(?m)^VERSION\s*:=\s*(%s)$`
@@ -32,15 +32,16 @@ const (
 type findVersion func([]byte) (string, error)
 
 var versionFiles = map[string]findVersion{
-	"versions.gradle":  versionMatcher(VersionsGradleRegexf, 1),
-	"build.gradle":     versionMatcher(BuildGradleRegexf, 1),
-	"build.gradle.kts": versionMatcher(BuildGradleRegexf, 1),
-	"pom.xml":          unmarshalXMLVersion,
-	"package.json":     unmarshalJSONVersion,
-	"setup.cfg":        versionMatcher(SetupCfgRegexf, 1),
-	"setup.py":         versionMatcher(SetupPyRegexf, 1),
-	"CMakeLists.txt":   versionMatcher(CMakeListsTxtRegexf, 1),
-	"Makefile":         versionMatcher(MakefileRegexf, 1),
+	"gradle.properties": versionMatcher(ConfigRegexf, 1),
+	"build.gradle":      versionMatcher(BuildGradleRegexf, 1),
+	"build.gradle.kts":  versionMatcher(BuildGradleRegexf, 1),
+	"versions.gradle":   versionMatcher(VersionsGradleRegexf, 1),
+	"pom.xml":           unmarshalXMLVersion,
+	"package.json":      unmarshalJSONVersion,
+	"setup.cfg":         versionMatcher(ConfigRegexf, 1),
+	"setup.py":          versionMatcher(SetupPyRegexf, 1),
+	"CMakeLists.txt":    versionMatcher(CMakeListsTxtRegexf, 1),
+	"Makefile":          versionMatcher(MakefileRegexf, 1),
 }
 
 func versionMatcher(regexf string, group int) findVersion {

newrelver_test.go

@@ -337,6 +337,17 @@ func TestVersionsGradle(t *testing.T) {
 	assert.Equal(t, "1.2.3", v.String())
 }
 
+func TestGradleProperties(t *testing.T) {
+	r := NewRelVer{
+		Dir: "examples/java/gradle.properties",
+	}
+
+	v, err := r.GetBaseVersion()
+	assert.NoError(t, err)
+
+	assert.Equal(t, "1.3.2", v.String())
+}
+
 func TestBuildGradle(t *testing.T) {
 	r := NewRelVer{
 		Dir: "examples/java/build.gradle",