Turn tuterm into a simple command interpreter (--interactive option)

[veracioux]

It would be very useful if we could run tuterm as an interpreter for commands.

An idea for the implementation:

• Create a temporary rcfile that will load all tuterm shell-functions/variables/etc. It will basically just source the contents of /usr/bin/tuterm (or equivalent).
• Run an interactive bash subshell with the -i option that will use the created rcfile

This would make it easier to debug tuterm scripts, and it would make it possible to create tutorials for how to create tuterm scripts!!