feat: make nyc instrument more clever about file extension

[bcoe]

it would be great if nyc instrument could accept a file extension when instrumenting files in a directory, alternatively when given a directory as input, perhaps we should just be applying nyc's include/exclude rules?

This feature grows out of errors that can occur if a user attempts to instrument an entire directory which contains files other than JS.

[JaKXz]

@AndrewFinlay Is this also covered in #893 or #977?

[AndrewFinlay]

I think you should be able to do this using glob patterns, which would be covered by #893. Using an include of **/*.js should do this for you, when it's all finished of course. #977 is more of a bug fix for the current --all command, where changes to the glob module removed support for !**/exclude.negated, so any exclude negated instrumentation candidates are being filtered out before they go through a second candidacy test in test-exclude.

[coreyfarrell]

I think we should use the extension array for this (so adding that option to the nyc instrument command).