Adds <mark-message> binding to create "hotkeys" for messages.

Adds <mark-message> binding to create "hotkeys" for messages.

<mark-message>foo<enter> will create a new macro "'foo" which will
return to the current message by searching for that message's
message id.  The initial character of the macro is defined by
$mark_macro_prefix, and defaults to "'" for verisimilitude vs. vi.

Pushed by Kevin McCarthy with a minor fix.