Add = (auto-indent) operator, x'd specs for it

The functionality seems to work, but this isn't ready to merge in
without some specs.
Add more tests for auto-indent, un-x previous spec

This changes the specs to use the JS grammer, as per @mcolyer's
suggestion. It also copies the format of the indent (>) and outdent (<)
tests by checking both multiple line indentation (2==) and the undo
functionality.
Reset the editor's grammar after each = spec
Add note in doc/operators.md about auto-indent
Merge pull request #125 from ciarand/fix/add-equals-operator

Add = (auto-indent) operator