Add --list-formats-supported-by-extension

[jidanni]

Feature description

User reads pandoc man page and gets all excited about a certain
extension, lets call it nerbowhich, mentioned on the man page.

Alas, at runtime the user gets a message:

The extension nerbowhich is not supported for html.
Use --list-extensions=html to list supported extensions.

User becomes curious, "well what formats are supported by nerbowhich"?

Therefore the above message should also say

Use --list-formats-supported-by-extension=nerbowhich to see supported
formats.

Also the man page should document --list-formats-supported-by-extension .

Oh yeah, and --list-formats-supported-by-extension needs to be written and added
in the first place too.