Completion support, OPML import/export

[FilipBB]

I was able to merge my recent feature additions with your new file layout. I also added the ability to parse urls with spaces or other funny coded symbols: f8c7c97

Fixes issue #7

[manolomartinez]

Thanks a lot. I'll review it as soon as possible.

[manolomartinez]

So, I was making tests with the opml command and stumbled upon a feed that does not have a "version" attribute, which messes with line 307 of commands.py:

Traceback (most recent call last):
  File "/home/manolo/Scripts/Oggregator/greg/venv/lib/python3.6/site-packages/f
    return self.__getitem__(key)
  File "/home/manolo/Scripts/Oggregator/greg/venv/lib/python3.6/site-packages/f
    return dict.__getitem__(self, key)
KeyError: 'version'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/manolo/Scripts/Oggregator/greg/venv/bin/greg", line 11, in <modul
    load_entry_point('Greg==0.4.7', 'console_scripts', 'greg')()
  File "/home/manolo/Scripts/Oggregator/greg/venv/lib/python3.6/site-packages/g
    function(vars(args))
  File "/home/manolo/Scripts/Oggregator/greg/venv/lib/python3.6/site-packages/g
    feedtype = aux.feedparser.parse(feedurl).version
  File "/home/manolo/Scripts/Oggregator/greg/venv/lib/python3.6/site-packages/f
    raise AttributeError("object has no attribute '%s'" % key)
AttributeError: object has no attribute 'version'

[manolomartinez]

Another thing I've noticed is that you have substituted sys.exit() with print in many instances. I'm fine with it, but at least I'd send that output to sys.stderr, thus: print("blahblah", file=sys.stderr, flush=True)

[manolomartinez]

I'd try to keep one convention on function names. I have_been_using_underscores, and you have added a number of camelCase functions.

[manolomartinez]

Still going through the PR! :-) Thanks a lot for the ton of work @FilipBB

[FilipBB]

Sure, thanks for the feedback Manolo. It's quite hacky because I don't have formal programming experience, so I don't know best practices. Let me know when you're done looking it over and I'll try to implement the changes you suggest and send a new PR.