1119648: Added additional functionality to repo listing.

Added the --list-enabled and --list-disabled options for explicitly
specifying only enabled or disabled repos. --list now acts as a
shorthand for specifying both.
I don't know if I'd bother now, but given the hoops required to test the stdout output, I think I would have been tempted to split the output formatting code into a method or two, so you could setup all the args/opts, then just test the method that formats the output and returns a string. Then this section could be:

```
if self.options.list:
    print self.format_repo_info(rl)
```