Add brolink.vim and vim-tmux-navigator

Brolink.vim is for live preview of HTML, CSS and JavaScript.
vim-tmux-navigator is for seamless C-hjkl between vim and tmux panes
Replace built-in grep with ag

When using :grep in normal mode, ag the silver searcher, will be
invoked. Also the '\' key can be used to grep, 'K' key to search
Add the vim-instant-markdown and clean the markdown settings

The vim-instant-markdown will be used to automatically open a browser
window when editing markdown. The settings are 80 width text.
Tweak the formatting of the gitcommit message

Although, this is still testing, thus commenting out the first line with
the formatoptions, I'll just leave it to textwidth 72 and the spell
check on.
Delete the C-hjkl to move around vim

This change is in favor of the vim-tmux-navigation plugin.