add support to pass title to link_handler

Hello,

Thanks for this great rss reader!

I'm using a link handler which stores links to http://getpocket.com but I would like to get the link titles there too. So I added the option to pass the titles in the link handler via `%t`, so for example in my case the link handler looks like this:

    link_handler("pocket-cli.py -a \"%u@%t\"")

If you like this change and want to merge it, I can also update the documentation accordingly,

Cheers