Add syntaxHintWithCommandHint: to ExMapping.

Given a command hint string (of the form bd[elete]), this
constructs a full syntax hint of the form:

  [range]s/regular expression/replacement[/[flags]] [line count]

The command hint, which is used in the spot where the
command name goes in the syntax hint, is passed in
externally because the part of the command name that is
optional depends on the other mappings in the map (since
characters are only optional if they don’t result in an
ambiguity between command prefixes).