What steps will reproduce the bug?

1. cp README.md README.mkd
2. bat README.md: shows file in all fanciness
3. bat README.mkd: no fanciness 😞

What happens?
mkd extension is not automatically handled as Markdown.

What did you expect to happen instead?
For all *.mkd files to be displayed as Markdown.
Although I stumble with them from time to time, looks like it's not an uncommon practice:
https://github.com/search?q=path%3A*.mkd&type=code

How did you install bat?
GitHub release

bat version and environment

$ bat --version
bat 0.24.0 (fc95468)