Use argparse; add option to specify JSON config file

There is a global dictionary named CONFIG. Initially it contains default
values, but a JSON configuration file can be provided using the
`--config` flag. The values set in the configuration file will overwrite
the default settings in the CONFIG dict.

Several function argument defaults changed to None (with a conditional
inside to use the default value from CONFIG) because the configuration
file is not read until *after* the function definitions are executed.
Support multiple STUN/TURN servers in config files

The support is very rudimentary. The configuration's structure can hold
multiple of each type of server, but the code that accesses the
configuration currently just uses the first one from the list.

This commit also adds a sample JSON configuration file.