Added kill-buffer function

Taken from
https://emacs.stackexchange.com/questions/3245/kill-buffer-prompt-with-option-to-diff-the-changes

TODO: Prompt for non-modified buffers
Fixed web-mode indentation of inline code
Only select buffers in current window
Aggressive-indent globally!

Still need to turn it off every now and then, but it is infrequent
enough that this is preferable.