weercd.py: use argparse module to parse command line arguments, remove config file

The configuration file weercd.conf has been removed. Instead, default options
can be set in an environment variable called "WEERCD_OPTIONS".
A file with options can be used, then name must be given as option with a
leading "@", for example: python weercd.py @args.txt

The option "action" has been removed. Default behavior is still to flood the
client. Actions "user" and "file" have been merged into a single option
-f/--file, which accepts a file, or special value "-" to read stdin.

The script now requires python >= 2.7 (because the argparse module is not
available in python 2.6 and older versions).