Added DOI regex function to utils.py which is called in add_cmd.py
upon specifying a new DOI. DOI validation applies directly on
argument parsing by using a custom argparse.Action.
Add a theme section in the configuration file to allow users to
set the colors used by different elements of the ui.
Improve the update mechanism so that incremental changes to the
configuration file can be incorporated.
`print_out()` is replaced by `message()`, that redirect the output to
the stdout controlled by the PrintUI instance. Other than that, can be
used the same way as `print()`.
