More involved vimrc for hhvm ondemands

[fredemmott]

• set background=dark - no one uses white on black terminals, right?
• tabstop=2, softtabstop=2: FB uses 2-space tabs
• expandtabs: ... actually, we use spaces

My preference, but hopefully uncontenious

• make 'backspace' allow deletion of more things
• enable mouse support in all modes

[fredemmott]

A bit more subjective, but I'm tempted to add:

• line numbers
• autoindent (not smart indent)