yoda: Allow extensions for generated bash completions

When an option accepts string/int argument w/o choices,
make it possible for the user to provice "ycomp_<name>_<options>"
function that will be called to generate possible completions
for an option.

E.g. for

option	foo
	string	foo option

int a project named "bar" a function named "ycomp_bar_foo"
will be called when user types

$ name --foo <TAB>

Signed-off-by: Pavel Emelyanov <xemul@sacred.ru>