Require --config to specify a file in the current working directory

[consideRatio]

If we don't constrain the --config flag to only work with files in the
current working directory, things won't behave as expected.

The project has been explicit about saying that paths resolve relative
to the config files that specify the paths, but if --config is allowed
to reference paths outside the current working directory chartpress no
longer honors its documented behavior.

Due to this, I think we should instead fail loudly than let users
experience weird behavior if --config file references a file outside the
current working directory.

[manics]

Could we os.chdir() into the directory containing the config file instead? The behaviour will still be unambiguous, and it's more user friendly.

E.g. os.chdir(os.path.dirname(os.path.realpath("config_file")))