Added deployTags option

Added new options & documentation to support auto creation of tags, and optionally pushing them to origin. From new docs:

>You may also provide a tag if you'd like to automatically create a new tag (and optionally push it up to origin):

 >```javascript
grunt.initConfig({
  pkg: grunt.file.readJSON('package.json'),
  'heroku-deploy' : {
      production : {
          deployTag : 'v<%= pkg.version %>',
          pushTag : true,
          origin : 'origin'
      }
  }
})
```
>This will create a new tag, push it to origin, and deploy that tag to heroku:

>    git tag v0.1.1  # create a new tag
>    git push origin v0.1.1 # push the tag to origin (this is skipped if pushTag is missing or false)

>Then everything else runs as if it were called with `{deployBranch : 'v0.0.1'}`