Recognize mailto URIs and don't prefix with file:

Fixes #373 (closed). mailto was not a recognized URI scheme and links with it were prefixed with file://, since they were assumed to be relative paths (the default behaviour). Adds mailto to the recognized URI scheme list to prevent the buggy behaviour.

There are many other URI schemes that may potentially result in the same problem and it may make sense to add them preemptively, though I didn't do that to keep the merge request minimal. The ones recognized by Pandoc are listed in https://github.com/jgm/pandoc/blob/f317ec41a1948e35330364c3120d937cc9934888/src/Text/Pandoc/Shared.hs#L887.