Add 'build_commands' option to specify build-time dependencies.

Note: This is a work in progress. I'm open to constructive feedback.

This pull request adds a 'build_commands' bundle option with the same format as the 'external_commands' option. Some plugins, such as YouCompleteMe and tern_for_vim require external programs at build time, or else the build will fail. I want to be able to specify those dependencies so that the build will be skipped without error if they don't exist on the host machine. It might be worth adding an option so that the plugin installation will give an error instead. I think it should be up to the user.

Looking forward to your review.