Adds support for Launchpad repos

I added this because I use some really good plugins that are primarily hosted on Launchpad, e.g. UltiSnips. Of course, Vim-Scripts mirrors vim.org and there are clones of various plugins on Github that are hosted elsewhere. But Vim-scripts only hosts proper releases, and other clones are often unofficial and not updated.

The changes are really small. I've added more note on the commit itself. Check it out.

Perhaps this doesn't properly "blend in" with the goals of Vundle, so feel free to deny this pull request if you feel this clutters things. I must admit that the reason for adding it is perhaps a bit shallow.