add support for matchesRegex placeholder

[Jazzyekim]

Introduce the implementation of matchesRegex placeholders. Need that functionality on the project I am working on. I decided to share my implementation with you as a PR
Since this placeholder contains a parameter I had to extend the signature of evaluate method to support parameters as optional array of strings.

[coveralls]

Coverage increased (+0.04%) to 91.808% when pulling fea1758 on Jazzyekim:placeholder-regex into 4866621 on xmlunit:master.

[bodewig]

Many thanks @Jazzyekim this looks interesting. It may also help #174 in a way.

Actually when I looked at your code first I wanted to see whether we could get away without breaking the interface with the help of default methods - even if that meant to move on to Java 8 for the placeholders project. Unfortunately I haven't found the time to try this out, yet.

[bodewig]

@Jazzyekim many thanks

I've rewritten PlaceholderDifferenceEvaluators argument list parsing a bit in order to make it more configurable in 22b014a - at least the existing tests still pass, except I had to change the one about the bad regex as the changed code would use the second parenthesis as opening the argument list and not cause any problem because the keyword then wouldn't match.

[Jazzyekim]

@bodewig, glad to hear that you found it useful! Many thanks