Support for .zip archives with unknown extensions at runtime

[jogleasonjr]

This allows for the listing of .zip archives with changed extensions, such as .docx, .pptx, .xlsx, .idml, and many others. An options.forceZip flag on the list() method enables this.

[edbrannin]

Issue: this only helps with list(), but I'm trying to use readFile() (which doesn't have an options parameter) on an .ipa (zip) -- I think it would be cleaner and more consistent if one of the following happened:

1. Expose a function that can add to the list of zip-like files
2. Expose readFileFrom* and list*()
3. EDIT to add: Check for zipness with something Magic-based

[jogleasonjr]

Expose a function that can add to the list of zip-like files

I did just that in my latest commit -- exposed an instance level function:

archive.configureExtensions(zip: ['.customZip'])

This way, one can add custom extensions for any of the archive types. It works with both list() and readFile(). There's a test for each.

My objective is to propagate this configurability forward into Atom's archive-view, so I can peek inside archives that have atypical extensions.

[edbrannin]

Works for me! (though I'm not a committer, just someone else with this need)