Make it possible for the theme to override the status line.

Make it possible for the theme to override the status line.

This is done by declaring a function format_status_line, which receives
the text that would normally be there (disk space left), as well as the
length of the current recording file in seconds. It can then return
whatever it would like.

My own code, but inspired by a C++ patch by Alex Thomazo in the Breizhcamp
repository (which did it by hardcoding a different status line in C++).