Highlight settings and LaTeX snippets.

- Map <F3> to toggle hlsearch.
- Some snippets for LaTeX
Fix rooter incompatibility with NERD_commenter.

When using NERD_commenter I like to use <leader>c to toggle comments but
vim doesn't proceed with the action until another key is pressed.

The problem is that the rooter plugin defines a mapping that interferes
with the NERD_commenter mapping, that is:

  map <silent> <unique> <Leader>cd <Plug>RooterChangeToRootDirectory

I don't use this feature so I commented this line and the problem is
solved.
Ignore Python Compiled files.
Remove mapping to create new tabs.

So far I have been using <leader>c to create new tabs but I want to use
that mapping to comment lines with NERD_commenter.