This is a second attempt at #104540, which is #63128 without dependencies.

This PR adds basic markdown formatting to rustc --explain output when available. Currently, the output just displays raw markdown: this works of course, but it really doesn't look very elegant. (output is rustc --explain E0038)

After this patch, sample output from the same file:

This also obeys the --color always/auto/never command option. Behavior:

• If pager is available and supports color, print with formatting to the pager
• If pager is not available or if printing with formatting to stdout fails, fallback to printing without formatting
• Follow --color always/never if supplied
• If everything fails, just print plain text to stdout

r? @oli-obk
cc @estebank
(since the two of you were involved in the previous discussion)