Add switch to supply additional go build tags

Add --build-tags switch to be able to supply additional go build tags

e.g. go-mutesting --build-tags=test,integration ./...

New mutator: structinit/remove

Example

	aVal := "a value"

	some := Some{
		a: aVal,
		B: 5,
	}

is mutated to

	aVal := "a value"

	some := Some{

		B: 5,
	}
        _ = aVal

and

	aVal := "a value"

	some := Some{
		a: aVal,
	}

Improved mutator

Improve the "statement/remove" mutator

Statements like "outer.inner = Inner{}" used to create broken mutations