Add optional custom git tag prefix
Pull `tag_prefix` into the Publisher initialiser.

We want to maintain backwards compatibility for the `version_released?`
method on `GemPublisher::Publisher`, as we sometimes use it in Rakefiles
<https://github.com/alphagov/govuk_template/blob/08d54d26/Rakefile#L69>.
Update documentation.

Change the documentation around tag naming, since this is now
customisable, and document the available options in some of the
`options` hash parameters (having gone spelunking in the code to find
out what options were available).
Add interpolation braces in Git tag string.

While it's technically valid to leave them out for instance variables,
it's less confusing and less risky to use braces on everything.
Merge pull request #12 from alphagov/custom_git_prefix

Add optional custom git tag prefix, version 2