Commit scope templating

You can use the scope template function to print the scope of a
commit.

A scopeheader filter exists to use custom scope names. Use the
scope_headers config flag as such:

<!--
Config(
  scope_headers: {
      "ui": "User Interface",
  }
)
-->

This feature is similar to the already existing type function.

See the default template file for an example of the
type feature, which should give you an idea on how to use scope and
its configurables.