automatically change session target based on buffer

One very confusion thing to me as I am very new to cabal was how targets work and it took me quite a while to figure all of this out.

It would have been a lot simpler (and would still be more convenient) if the session could be automagically changed depending on which buffer you're in.

haskell-mode should be able to figure out which component a buffer belongs to by comparing its path with the components' `hs-source-dirs`.