Export in a format compatible with cheat

[veracioux]

I think it would be great if we could export tuterm scripts in a format suitable
for the cheat program. To enable exporting, the user should pass a
-x/--export option or similar.

Even then, not all commands should be exported, but only specific ones. I
suggest that we add a cmdline-export shell function along with an alias
C or cx. Only command lines specified using this function should be exported.

To export messages, they should be declared using message-export or
Message-export instead of the usual message and Message respectively.
Naturally, two new aliases would be added: mx and Mx. Alternatively,
exportable messages could be declared using the usual message/Message by
passing a -x option. In the latter approach, parsing the -x option could
prove painful, so I am inclined toward the former, which is also consistent
with the cmdline-export approach.

Note that even if a commandline or message are declared as exportable, they
should not be exported unless tuterm was called with the --export option.

The following question arises: should we allow a tuterm script to both be run
and exported at the same time? I think we should, but we should also add a
-r/--run option. If --export is absent, --run is not necessary, as the
script will run anyway. If the --export option is given, the script will run
only if --run is specified.