Make .clanvasrc file location configurable.

[rowdyrotifer]

Currently, the .clanvasrc file is hardcoded to the user's home directory (~/.clanvasrc)

It would be nice if there were ways to specify alternate locations for those with custom configurations.

Add a function that resolves the path of the rcfile by using an approach with a series of fallbacks:

1. Check if the user specifies a flag called --rc-file, if they do, use that option's value.
2. If they do not specify that flag, then check an environment variable called CLANVASRCPATH, if they do, use that option's value.
3. If they do not specify that environment variable, then simply use the current value: join(expanduser('~'), '.clanvasrc')

Use argparse. ArgumentParser to read the command-line arguments, and os.environ to read environment variables.

[rowdyrotifer]

Instead of this approach we'll go with the method described by issue #6 which gives more flexibility for other files as well.