Add config file support

[hololeap]

Some future features may be assisted by having a config file, so that a user can change default settings for haskell-updater.

It will have to depend on only packages bundled with ghc so that it doesn't pull in any more dependencies. This means we may have to roll our own config format or use one provided in e.g. Cabal.

Some ideas for where to keep it:

• .haskell-updater/config
• .config/haskell-updater-rc
• .local/share/haskell-updater/config