allow deletion of tags

It would be nice if I could delete tags. Currently I have a release process that reads the version number from a version.json file and automatically creates the tag locally and then pushes it:

```
git tag releases/x.y.z
git push --tags
```

These 2 commands can be achieved currently with `grunt-git`

Occasionally I make a mistake and would like to undo this :o 

```
git tag -d releases/x.y.z
git push origin :releases/x.y.z
```

The second command I can do with `gitpush` but the first I cannot do with `gittag`. I tried setting the `tag` option to `-d releases/x.y.z` but this displeases the spawn call (I don't know why).

This pull request adds a `remove` option to `gittag` to add the `-d` to the `args`  array.

I chose `remove` for the option name instead of `delete` as `delete` is a keyword and although probably fine, gets highlighted as such in editors