A top-level font now has a list of fallback fonts. When a glyph cannot
be found, we try each fallback font in turn, until we either find one
that has the glyph, or until we've exhausted the list.
To make this actually work in practise (read: to make performance
acceptable), the cache is re-worked and is now populated on demand.
It also supports non-ASCII characters, by using the 4-byte unicode
character as index instead.
Since having an array that can be indexed by a 4-byte value isn't
really viable, we now have a simple hash table instead of an array.
The default is to reverse the foreground/background colors in the cell
with the cursor.
But, if the user configures a specific set of cursor colors, those
will always be used, regardless of other cell attributes (dim, reverse
etc).
The cursor color is specified as two color values, 'text' and
'cursor'.
The block cursor uses the 'cursor' color as background, and the 'text'
color for the glyph.
All other cursor styles uses the 'cursor' color for the cursor, but
uses the cell's foreground color for the glyph (meaning,
dim/reverse/etc applies).
* Strip whitespaces from keys and values
* Detect (and ignore) comments
* Detect syntax errors (no value specified etc)
* Error out on syntax errors and invalid keys
