Adding BetaLabelInserter

A new Inserter Class for adding the Beta Label style. This will eventually replace the Beta Notice as the de-facto way to show Users that a page or service is in Beta.

To use this, send a  CSS selector with a position like so:
`[Slimmer::Headers::BETA_LABEL] = "before:main.content"`

We currently only accept before and after as the 2 options for where it can be inserted.

Counterpart to https://github.com/alphagov/static/pull/383.

This follows on from https://github.com/alphagov/slimmer/pull/64 and https://github.com/alphagov/static/pull/381.