Use best monospace font for every OS + fallback

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

If an OS user installs a font from another OS,
they will of course get this font.

If none of these fonts are available to a user,
they will get the default monospace font
according to the "monospace" font family name.
Merge pull request #207 from vassilevsky/monospace-fonts-fix

Use the best monospace font for every OS