Added options to tag method.

Adding a tag through base.tag takes an optional hash of options.
* sha=>[sha commit id] 	- adds tag to given commit
* force=>[true|false]	- adds -f option to command, will overwrite an existing tag with the same name.

Usage:```
repo.tag("tag_name_1")
repo.tag("tag_name_1", {:force=>true})
repo.tag("tag_name_2", {:sha=>12345678910})
repo.tag("tag_name_2", {:sha=>92345678911, :force=>true})
```