Initial support for terminal within micro

Initial support for terminal within micro

This commit adds beta support for running a shell or other program
within a micro view.

Use the `> term` command. With no arguments, `term` will open your
shell in interactive mode. You can also run an arbitrary command
with `> term cmd` and the command with be executed and output
shown. One issue at the moment is the terminal window will close
immediately after the process dies.

No mouse events are sent to programs running within micro.

Ref #243