Use the best monospace font for every OS

Hello again :)

I gave it another thought and have come up with this:

Users of all major operating systems must get the best fonts for theis OS:

1. Windows - Consolas
2. Mac OS  - Monaco
3. Linux   - Ubuntu Mono

Of course there are more good monospace fonts in every OS and in general. But if we include them all, the string will be too long :) I picked what I believe are the best fonts; the `monospace` fallback should cover the rest (users can pick their favorite monospace font in their browser settings).

I hope that this version is better.

Thanks!