Leave room for the status bar at the bottom.
Add buf_printf function.
Add editor.status field and draw it.

Show "-- INSERT --" in insert mode, empty it in normal mode.
Show file path / size in status bar on opening.

Along the way, make some changes to file_t:

* Rename file_read to file_open
* file_open now assumes file exists -- let the caller make sure
* Add file_create to create an empty file.
* Add file_size function to count the characters in a file.