Accept a theme with the source code, return a new theme if modified

[johno]

This requires us to set the theme that's passed in for the canvas and rendered blocks (which I believe we might be doing) and then handle the theme change by calling onThemeChange when the theme panel is edited.

The API would look something like the following:

<Editor
  src={myJSX}
  theme={myTheme}
  onChange={handleJSXChange}
  onThemeChange={handleThemeChange}
/>

[joeynimu]

@johno Hi, I'd like to take this one on if you can guide me if that's okay with you.