allow multiple fallback fonts

[dylex]

Related to issues #3 and #4, allow a list of fonts and fallback to subsequent fonts when glyphs aren't defined, like urxvt. This does sound like a lot of overhead, but will relieve the need for unicode fonts.

[dylex]

Alternatively, we could pre-combine the fonts, which is what I seem to have done (maybe I should commit the script I used for this). This does make a lot more sense in terms of efficiency, but perhaps not in terms of memory usage.

That said, I'm getting a better idea how to implement the checking now in the case where we can fall through by attribute per-character.

[dylex]

8d82f1e does this now for the attributes, so if bold/italic fonts are missing something, it can fall back. Adding additional fonts would not be terrible, but I'm not sure I want to do this.