Move default keybindings including shell commands to external tigrc file

The tigrc file is install to ${sysconfdir} by `make install`. To ignore
the default keybindings and load a different set of keybindings, for
example keybindings that do not allow to run commands that modify the
Git repository, use something like:

	TIGRC_SYSTEM=~/.tigrc.safe tig

This also removes the built-in `git gc` keybinding, which was never
really that useful other than scaring new users.

Fixes #235
Add tigrc copy as a fallback inside the binary
Move default color settings to tigrc
Move default settings to tigrc

Add all default values for settings to tigrc with documentation for each
of them.
Delete contrib/tigrc now obsolete by tigrc