widgets: add a spellcheck widget

This is an implementation of Peter Norvig's "How to Write a Spelling
Corrector".

http://norvig.com/spell-correct.html

The widget highlights typos and offers suggestions.

Signed-off-by: David Aguilar <davvid@gmail.com>
commit: add an inline spell checker

We can now check spelling using either "enchant" or a built-in Norvig
spell checker.

Signed-off-by: David Aguilar <davvid@gmail.com>
doc: update release notes

Signed-off-by: David Aguilar <davvid@gmail.com>