Support :exec in REPL

Now, the result of :exec is opened in a Comint buffer. This relies on:

https://github.com/idris-lang/Idris-dev/pull/1018

but is backwards-compatible.
Add command and menu item for :exec

Support executing programs from the menu, and with a M-x command.
Merge pull request #107 from david-christiansen/issue/106

Support :exec in REPL