Add menus for interpreter options

Now, settings in the running interpreter can be modified using
menus. Thus far, only showing implicits and error context are supported,
but the system is straightforward to generalize.

This is not done using customize because the typical use case is to
briefly toggle them while debugging, and persistency is not desired.
Merge pull request #102 from david-christiansen/options-menus

Add menus for interpreter options
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