Cross-validation script: Add filter for extensions

[DecimalTurn]

Allowing the use of the cross-validation script with an extensions filter to make the script run faster.
Especially useful when testing multiple samples.

Related comment : #6476 (comment)

It would help if I could run the test script on only the .bs files (or only the Bluespec BH directory) -- any advice on how to do that?

It's too late for this PR since the sample-related issue is resolved, but I had to do this recently as well when having to fix a classifier issue with namely .bas and .cls files. The branch with that feature is this here https://github.com/DecimalTurn/linguist/tree/cross-validation-filter

The syntax to apply the extensions filter is to pass the --extensions option with extensions (including the dot) seperated by a comma.

bundle exec script/cross-validation --extensions=.bas,.cls
or
bundle exec script/cross-validation --extensions=".bas, .cls"