vimrc: Jump to next (previous) split with 'gs' ('gS')

...not to mask the original Tab mapping.
vimrc: Use vim-easymotion instead of vim-sneak
vimrc: Replace vim-pathogen with Vundle

It also automates cloning the plug-ins from github.
vimrc: Install the vim-surround plug-in

Key bindings:

cs<old-paren><new-paren> -- change surrounding parenthesis
ds<paren> -- delete surrounding parenthesis
ys<textobj><paren> -- add surrounding parenthesis
S<paren> -- add surrounding parenthesis in visual mode

https://github.com/tpope/vim-surround
install.sh: Install Firefox extensions

Unfortunately it involves manually confirming each add-on in Firefox.