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
Add a major mode for .ipkg files
Add completion to idris-ipkg-mode
Find the ipkg file automatically
Add major mode for Idris build buffers

This gives a keymap for quitting with q
Menus and keybindings for ipkg
Add keybindings for package operations

Now, idris-mode proper has keybindings for package operations.
Add new `covering` keyword
Merge pull request #108 from david-christiansen/idris-ipkg-mode

Major mode for ipkg files
Merge pull request #109 from david-christiansen/covering-keyword

Add new `covering` keyword