Add option to skip errors when no matches are found

[roqvist]

Adding a new option --allow-empty that, when supplied and no matches are found, prints the "No matches" -information and then exits without error (code 0).

My use case is similar to #104 where I run ruplacer as a pre-build task on some source code, and it typically only replaces when someone else made changes. I don't want subsequent attemps to necessarily throw errors, as the replacement has already been made.

Fixes #104

[dmerejkowsky]

Looks good to me :)