Support highlighting metavariables

Support Idris's semantic info on what names are metavariables and use
the same face as in source code
Support docstrings and types from ideslave

IDESlave, following the merge of
https://github.com/idris-lang/Idris-dev/pull/1014 , will provide clients
with documentation overviews and type signatures. This shows them in tooltips.
Merge pull request #103 from david-christiansen/metavar-highlight

Support highlighting metavariables
Merge pull request #104 from david-christiansen/docs-n-types-in-output

Show documentation and types in Emacs
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
List metavariables on file load

This relies on https://github.com/idris-lang/Idris-dev/pull/1046

Backwards compatibility is achieved by disabling the hook.
Fix incorrect synopsis
Merge pull request #110 from david-christiansen/metavariable-list

List metavariables on file load