add Vimcasts and exercises to tutorial section

[juanMarinero]

The tutorialrc is very good. This PR is a complement.

I coded vim-visual-multi-exercises to apply Learning-by-doing, details how in this FAQ.

This PR also provides some vimcasts that I think are quite useful for learning VM.

Thanks for this awesome plugin, and for your time!!

PD. License. I hope that my vim-visual-multi-exercises - license is right, compatible. If it's not, then I am sorry. Please guide me how to fix it. Thanks!