Add lisp alias for common lisp

[bonniee]

Noticed when debugging my jekyll + github pages setup that rouge doesn't highlight code blocks marked as "lisp".

fwiw, this matches pygment's alias for common lisp as well.

I'll push up a branch to add an alias, if you'd be open to it. Thanks!

[stale]

This contribution has been automatically marked as stale because it has not had any activity for more than a year. It will be closed if no additional activity occurs within the next 14 days.

[pyrmont]

Sorry, marking this as stale was a mistake. The issue has an associated PR and should remain open. I'm sorry it's taking a while but we are slowly making our way through the backlog and will get to #848.