Support text attribute if no title was provided + argument usage example

[e41q]

I ran into problem, that exported .opml file from airsonic doesn't work correctly with podsnatch. After small debug, I reach out that it uses text attribute for outline tag, instead of using title.

I made a support of text attribute if no title attribute was provided. And for critical case, if no title and text attribute specified - use last part of provided url.

And documented use of --number-of-episodes argument.