Fix build failure against haskell-src-exts-1.14 (PTuple got boxed argument)

$ cabal sandbox init; cabal install ./lambdabot-haskell-plugins ./lambdabot-core/

[19 of 33] Compiling Lambdabot.Plugin.Haskell.Undo ( src/Lambdabot/Plugin/Haskell/Undo.hs, dist/dist-sandbox-e2d62edd/build/Lambdabot/Plugin/Haskell/Undo.o )

src/Lambdabot/Plugin/Haskell/Undo.hs:83:14:
    Constructor `PTuple' should have 2 arguments, but has been given 1
    In the pattern: PTuple ps
    In an equation for `irrefutable':
        irrefutable (PTuple ps) = all irrefutable ps

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Added cabal.config to .gitignore