Issues with command state/value overrides 

So, I've noticed that in the example editor, hitting bold, italic, etc before typing doesn't always work as expected.

If you type a few non-space characters, and hit command-B, the *next* character you type will be bold. However, if you type a space character (translated to an &nbsp;), and hit command-B, it appears that nothing happens.

After some investigation, I believe that scribe is inadvertently causing the [contenteditable state/value](https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html#state-override) overrides to get reset, probably while running formatters.

I've written a small test to demonstrate this, and have temporarily patched the issue in command-patch.js, by just not running the transaction manager on an empty selection. Perhaps there's a better place to make this change? Would love some feedback on this.