Deploy to heroku via tags

Right now we create a tag at master previous to deploy. This was
initially used to track where our apps were at in different
environments. When the deploy happened it was always pulling from
master. I would like to see the deploy happening from a reference
point... like a tag.

Use case:

I tell paratrooper to create a staging tag which will tag current master
and deploy to my staging app.

Then when I tell paratrooper to deploy to the production app I would
like it to create a production tag at the reference point of the staging
tag and also deploy from that point.

This way we don't run into the problem of deploying to production
straight from master. I run master under the assumption that everything
is working but not necessarily confirmed as correct by the client.

Looking for api suggestions

Currently you pass a tag in like so:

```ruby
Paratrooper::Deploy.new('app', tag: 'staging')
```

Thinking something like this
```ruby
Paratrooper::Deploy.new('app', tag: 'production', deploy_tag: 'staging')
```