metalsmith.match() issue with metalsmith-branch

Something I've noticed in my own plugins @webketje is how `metalsmith-branch` will only pass a subset of files to a child plugin, but if you use `metalsmith.match()` it will match against the "global" list of files, and you will end up with some filenames that might not be in your "working" file list.

The fix is using `metalsmith.match(pattern, Object.keys(files))` in every plugin, like this: https://github.com/emmercm/metalsmith-reading-time/pull/88/files#diff-92bbac9a308cd5fcf9db165841f2d90ce981baddcb2b1e26cfff170929af3bd1R16

Review the plugin implementation for every use of metalsmith.match(), starting with the behavior described for metalsmith-branch. Use the linked metalsmith-reading-time pull request as a reference. Done means matching is restricted to Object.keys(files), so a child plugin receives only filenames in its working file list.

Written by the indexing model from the issue text.