Make check for external dependencies extendable?

[jzaefferer]

Core currently requires md5sum, UI requires unzip. Arguably those could all be replaced with node modules. Both UI and Mobile have their own implementation for creating md5 sums. Last I checked, there was no decent unzip implementation in node (archiver only does zipping; node's zlib wrapper only handles compression, not the actual archiving).

If we make the check extendable, we'd have move or duplicate it, since it can only run after _cloneRepo.