Move JSON filetype detection to ftdetect directory

To keep things organised, use the .vim/ftdetect directory instead of my
vimrc to set the JSON filetype.
vimrc: disable default ftplugins

Only run my ftplugins, not the ones that vim provides.

NOTE: this doesn't work. Figure out why.
vimrc: disable default ftplugins (working)

Working version of disabled default ftplugins.
vimrc: move gitcommit settings to ftplugin

Move gitcommit indentation settings from vimrc to an ftplugin file.
Keeps things more organised and makes for a cleaner vimrc.
twilight256: colour inactive statuslines dark grey

Use a dark grey colour on statuslines of inactive windows to make it
easier to differentiate between active and inactive windows. The active
window remains the old light grey.
ftplugin/gitcommit: set textwidth=72

Actually enjoyed the textwidth setting of the default gitcommit
ftplugin so bringing that over to my custom one.