don't markdownlint untracked/ignored files

I like to put a bunch of untracked stuff in a /scratch directory for each project, and then puth /scratch in my global gitignore. There are usually some markdown notes files in here that I don't care about style for. The previous markdownlint invokation didn't respect the global gitignore, making local 'engage' runs kinda useless due to false positives from the scratch dir.

This is slightly different semantics from the other linters, which will skip gitignored files but will not skip untracked files.