Implement ExcludeTags, with tests. Fixes #231

I think @fearthecowboy already submitted a fix for #231, but I already did the work anyway, so here's mine. You can compare them for fun. I wrote a few extra tests (pulled from @dlwyatt's LanguageDecoupling branch) and I have the changes for Gherkin and output.ps1 to apply on that branch, on my copy of the LanguageDecoupling branch ;-)

Looks like the main difference between my fix and @fearthecowboy's is that I called the parameter ExcludeTagFilter  (and injected it in the parameter list, instead of adding it on the end), and wrote the extra tests in Describe to verify the functionality.