Add option for files that should be bundled, but not parsed/walked

_Despite this being the 3rd pull request for this, I promise I'm not trolling you or being a pest (on purpose at least). It was just dumb to open the request against my master and dumber still to change it to a particular commit. Hopefully using a feature branch is less dumb._

I love browserify b/c it's awesome :sparkles:, but building was being a little slow when crunching big files like jQuery, Underscore, and Backbone. Caching helps on subsequent builds, but (and maybe I'm being ocd) I _know_ those files don't need to be crawled, so I hate to waste cycles doing it.

This seems like what `ignore` would do, but that option leaves the file out of the resulting bundle. That seems like a helpful option, and folks are already using it as-is. So I'm proposing `noParse` as a complementary option to include a file in the bundle, but skip crawling its AST.

I wasn't using browserify before #90 and caching, but they seem to have made a significant difference. I can verify anecdotally that using `noParse()` helps even more, particularly on the first build.

Is this a feature you want in browserify? If not, I'll let it die. If so, I'm more than open to re-implementing it differently if need be.

Thanks, @substack!