Added basic settings functionality

While settings can be properly loaded/saved, they are not actually applied;
need to interface with codemirror to apply them.
Merge branch 'master' of https://github.com/cs136/seashell
Replaced any/c with jsexpr?
Fixed contract bug in files.rkt

Needed to require json for jsexpr? predicate
Merge branch 'master' of https://github.com/cs136/seashell
Added some support for the settings dialog

Made the tab width option work, and also added a theme option, with several
available themes which come with CodeMirror. Currently, the font size, editor
mode, and expandtab (spaces instead of tabs) options do not work.