Create new submodule update target to run git submodule update

Resolves #59 at least partially. I have no need for the other submodule commands, but can start adding them if you would like.   

My current project needed to perform submodule updates during the build process, so I added this functionality into grunt-git. I tried to maintain the current style, but did do one thing very differently: Instead of pushing options onto the args array individually, I do so in a loop that loops through an array of ``acceptableOptions``. This way, only options that we are expecting will be passed to the child process, and those that might cause an error (e.g. ``verbose``) will not be. 

**I'm happy to convert it to the more explicit method used elsewhere in this repo if you would prefer.**

All CLI flags for git-submodule-update are supported by this patch, though I suspect that most folks will only use *init* and *recursive*. 

I've also updated the README to include documentation for the new task, and added tests for each option.
See the note on case above, we can probably do this automatically with some regex magic.

Something like this:
```js
"noFetch".replace(/\W+/g, '-').replace(/([a-z\d])([A-Z])/g, '$1-$2').toLowerCase() -> "no-fetch"
```