Allow to enable offline mode

I had pretty much explained the reason in the README, just to clarify some things here. In my case it's about Windows and Git client being used through Git Bash instead of adding into PATH environment variable.

I am using [grunt-browserify-bower](https://github.com/tobiash/grunt-browserify-bower) which is dependent on this package. Running the grunt task in Windows CMD simply fails with Spawn ENOENT error. When I use Git Bash, everything works fine. I had to debug whole thing to find out the culprit.