gitk: add proc run_themeloader

gitk: add proc run_themeloader

gitk currently accepts a single themeloader file via the config file,
and will source this with errors reported to the console. This is fine
for simple configuration, but will not support interactive theme
exploration from the gui. In particular, a themeloader file must be
sourced only once as the themes defined cannot be re-defined. Also,
errors must be handled rather than just aborting while printing to the
console.  So, add a proc to handle the above, supporting expansion of
the gui config pages.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>