Add initial version of grabber, prototype xkcd

This is a very early prototyping version of the grabber script, which simply
gets the first 10 image urls from xkcd (only the urls, not the images).
Make code more general; options; element spec

Start making code more general. I understand how to use BeautifulSoup to grab
the contents of a web page, so now it's time to build a program around it.

First of all, I added some program options using the argparse module provided by
Python. This is a real blessing and makes working with command line parameters
very easy.

Then I started the parsing of a specifier string for the interesting HTML
elements. For now, you can extract a string from the HTML by using the specifier
format <CSS selector>[/<attribute name>], which will select the element by its
CSS path/selector; if given, the content of the specified attribute is used; if
this is not given or empty, the tag's string content is used.
Move extraction of element into function
Add more options and help text
Move grabbing into function, add args checks
Remove old, commented-out code
Add using of next url and end-of-comic detection
Add writing of images to disk