Cleanup the save box size code
Use Chrome commands instead of custom key commands

- Alert the user if they use it on a chrome:// url. Closes #9
- Alert the user if they use it on a file:/// url. Closes #7
- Set the default chrome key command to Shift+Ctrl+E. Closes #8
- General code cleanup and linting

This commit also makes the box size save properly if the user uses
the key command to close the editor because of the way that the
browser_action commands work. Per #1
Merge pull request #10 from jhubert/key-commands

Use Chrome commands instead of custom key commands