Add a --setting-overrides KEY=VAL commandline option to override
arbitrary items from pelicanconf.py settings files. This adds
flexibility in running pelican and helps reduce sprawl of settings
files. Cast int and str setting overrides to their respective types. Support other setting types by treating them as json. Fallback to json when an override typecast errors. This should make it
possible to set int values to None, resp. to json 'none'
This commit removes Six as a dependency for Pelican, replacing the
relevant aliases with the proper Python 3 imports. It also removes
references to Python 2 logic that did not require Six.
