Every time the scrollbar reaches the end, we append 100 more rows
(seamlessly!).
As a result of defaulting to 100 rows, we can also save with the
smallest number of rows required.
This partially deals with #4170.
Currently only supports setting the foregound and the background colours.
This patch also unifies `foreground_color' and `background_color' used
throughout to a `Format' struct, in hopes of getting more formatting
options one day :P
This commit adds a generic interface for cell types and hooks it up.
There is no way to set these from the UI, and so they're not saved
anywhere yet.
Also implicitly converts numeric values (strictly integers) to numeric
javascript values, as numbery-looking + numbery-looking === string is
not very interesting. :^)
