Add fswitchfnames option that allows mutating the base filename when looking for a companion file.

[thesamet]

Hi Derek,

I've implemented a new feature (that is useful at least for me) and hope that you can integrate it.

The idea is that I would like to use FSwitch for switching between implementation and test code. For SomeFile.scala my test file would be called SomeFileTest.scala. It would also be in a different directory (but b:fswitchlocs takes care of that). So I thought to introduce a new option that allows substitution over the base filename. Here is how I use it:

au FileType scala let b:fswitchdst = 'scala'

" Matches scala files that do not end with Test.scala
au BufEnter *\(Test\)\@!.scala let b:fswitchlocs = 'reg:+/main/scala+/test/scala/+' | let b:fswitchfnames='/$/Test/'

" Matches scala files that do end with Test.scala
au BufEnter *Test.scala let b:fswitchlocs = 'reg:+/test/scala+/main/scala/+' | let b:fswitchfnames='/Test$//'

[derekwyatt]

Cheers. I'm going to merge this in after marginal testing on my part. With luck there are no issues.

[thesamet]

Thanks