Handle double quotes intelligently

If there is a selection, surround it with `` and ''.  Insert '' if
following a word; otherwise insert ``.  If following a backslash, just
insert ".  Closes #15.

Also, make $ into a repeat pair for easier equations.
Fix typo in copyright statement
Better handling of parenthesis and brackets

Their prefixes are now mirrored on the closing characters.
Enable external PDF viewer

The settings gain a "PDF-viewer" item, which can be set to "internal to
use the built-in viewer, or to a command to launch an external viewer.
The current cursor position is available, for SyncTeX-aware viewers.
When using an external viewer, errors are written to the console within
Light Table.

Closes #16.
Version 0.0.9