Set version number using git-describe

Adds the `$ grunt bump:git` command which generates a version number using [git-describe](https://www.kernel.org/pub/software/scm/git/docs/git-describe.html). 

Default command is `$ git describe --tags --always --abbrev=1 --dirty=-d`, but cli options can be set in the grunt config:

```js
bump: {
  options: {
    // ...
    gitDescribeOptions: '--tags --always --abbrev=1 --dirty=-d'
  }
}
```