Add liquidprompt support, if detected
Squashed 'src/liquidprompt/' content from commit 7f54e6d

git-subtree-dir: src/liquidprompt
git-subtree-split: 7f54e6dee797be5f3ae4a737475fc73bdb165fc2
Merge commit '49f21b11a24550dd8afa9109d832e4d67bc437da' as 'src/liquidprompt'