[vim] Disable persistent undo from vim-sensible

The persistent undo feature causes `.un~` files to be scattered around
the filesystem. This could be fixed with an "undo directory", but I
think that it could cause more harm than good anyways, so this disables
it altogether.
[ghci] Add configuration file for `gchi`

`ghci` is a command-line Haskell REPL. This ensures some extra packages
get imported by default, and we now have a pretty unicode lambda symbol
for the prompt. Ideas were stolen from
<http://dev.stephendiehl.com/hask/flat.html>.