Fix fallback font

Using `'Monospace'` as fallback font seems to not work properly. This PR replaces it with `monospace`.

A thing to keep in mind is that if the `font-family` is defined in a theme, people can't pick their own by entering it in the settings. I advise to remove that line and change the README to something like this:

"Quantum works best with the `Source Code Pro` font, if you have it installed you can enable it in Atom's settings by enter the name into the Font Family field".