Optional tagging

Hi, thanks for the awesome grunt plugin, it does almost everything I need for distributing a compiled version of my project into a public repository.

I've added a feature to my fork that may be useful: tagging the commit for bower/component version resolving.

In the implementation if tagging fails then it does not fail the whole task because this tag probably already exist in the repository.
This way I can run this task in my continuous integration build system bumping the version only when a significant change was made.