Add :save-display command

This allows to dump the current ncurses screen data to a file. The main
use is for testing. The command optionally takes a file name.
Add support for running scripts using :script <file>
Add TIG_SCRIPT and TIG_NO_DISPLAY environment variables

The former allows to execute prompt scripts automatically on startup and
the latter forces Tig to not render anything to the terminal.
Add test framework

This adds a small test framework and updates all existing tests to use
it. Tests are now implemented using shell scripts and integration with
the build output is much improved.
Add test for tigrc compat errors
Add test for the main view