Display buffer should not be recreated each update

Currently the display buffer is recreated each update. Kilo's default implementation mallocs a new 0 length buffer, uses realloc *every append* and frees the buffer when it is done.

Ideally, the display updater is passed a buffer that it can overwrite and re-use. The buffer can be longer than required so long as the number of characters to write is tracked and passed into ```write(...)```.

A set-length self-flushing buffer could be used so long as *appends* are partially written if they exceed the buffer limit. Probably needs a unit test.