Add Key, test for printable character on keydown when selection

[bantic]

Uses a whitelist of keycodes to determine if a character is printable.
There will probably need to be additions to that heuristic in the future,
but this feels pretty reasonable for a first approach.

This fixes the issue of content editable adding garbage HTML when you select two text nodes
that have different styling (e.g., part of a heading and part of a plain-text section) and
hit a character. Chrome inserts a span to try to preserve the styling of the plain-text
when it removes the selection and joins the plain-text to the heading.

Fixes #50

cc @mixonic