Define default fonts

[arhpreston]

A few minor improvements:

• Sensible default fonts (Times New Roman and Arial Bold) so users don't need to add any config to get up and running
• New method get_font to allow users to specify just the font name (rather than full path to font)

[bettsmatt]

Sensible defaults are good. I'm not sure trying to find where fonts are installed per platform is the best approach. Could we include fonts in the package instead?

[arhpreston]

Could we include fonts in the package instead?

I guess you could package free fonts. Coding the paths for different distros works fine too. Your call.

[bettsmatt]

Tried to merge and it broke.

• I think the platform should be .startswith('linux').
• Error messages are now not as friendly when the config is missing and the platform is not recognised (runtime vs startup error).
• Scanning multiple directories will slow everything down (this should be moved to some kind of set up anyway).

Alternatively we could check for system fonts when the module is loaded and then throw an exception after loading if none are found. We could also read these or a subset of them in to prevent looking up the font in every request.