Reduce conversion of Symbol to String

[ashmaroli]

Instead of interpolating :rouge (default and therefore more common scenario) to "rouge" multiple times when a Kramdown::Document is converted to_html, use a frozen_string to be used per instance.

[gettalong]

Thank you for your contribution!

If you can adapt your pull request as per my comments, I will merge it.

[gettalong]

Perfect, thank you! I have squash-merged your changes and additionally, since the new @highlighter instance variable is only used in one place where it gets interpolated into a string, I have changed the instance variable to do the interpolation only once, saving additional string operations.

[ashmaroli]

have changed the instance variable to do the interpolation only once

That's even better!! ✨