Auto size font on overflow

[okaybenji]

Suggesting shrinking font when text overflows. This fixes issue which causes rendered CanvasInput to be out of sync with the actual input box in the following situations:

• Cmd+Arrow / Home / End key text cursor navigation
• Arrow key text cursor navigation
• Repositioning text cursor with mouse click
• Refocusing on CanvasInput with mouse click

Fixes #4

Note: Please be sure to update semver on this branch before merging.

[goldfire]

I think this may require some more thought since the goal of CanvasInput is technically to mimic a DOM input. After playing around with this, it seems to cause UX issues in some cases. However, this could be good to add as an option since I could see the benefit in some scenarios.