Add link to sheerun article in TODO comments, too

Add link to http://sheerun.net/2014/03/21/how-to-boost-your-vim-productivity/ to remind me to add everything useful from it
Scroll horizontally, too, to keep cursor visible

with 'sidescroll' and 'sidescrolloff'

from https://github.com/sheerun/vimrc/blob/36db540eeb7c91293be51dbc126490e8c3b7fdab/autoload/vimrc.vim#L155-L157
Make Backups section separator consistent with others
don’t display the intro message on starting Vim

from https://github.com/sheerun/vimrc/blob/36db540eeb7c91293be51dbc126490e8c3b7fdab/autoload/vimrc.vim#L270-L271
Change space-space mapping to leader-space

So it’s clearer that leader-space is taken
Change from Command-T to ctrlp.vim

Because Command-T requires Ruby, which isn’t available on all versions of Vim, such as gVim for Windows
Change introduction text to reflect that I have changed a lot

And add the two most recent sources
Update tagbar plugin

The repo had gotten dirty somehow, and a Git pull fixed it.