support cwd

[JaminShanti]

I have a need to issue the git tag commands in a particular directory different than the directory where the grunt commands are issued. Can you add cwd support to grunt-bump.

[eddiemonge]

Initially I was going to say no to this as it such an odd edge case but implementation will probably take less time than writing this comment. Ok maybe not but its probably not far off. All thats needed is to pass an options object to the exec commands that takes cwd if it exists.

var execOpts;
if (options.cwd) {
  execOpts.cwd = options.cwd;
}

exec(cmd, execOpts, cb...);

Something like that. If you want to submit a PR, then go ahead and there is a high chance it will get merged.