Allow giving options through input pragmas...

[mgajda]

Cabal still doesn't support giving custom Alex options in configuration file (see issue: https://github.com/haskell/cabal/issues/1223), and some programs need to specify their own --template directory, or use predicates that are as of recenlty incompatible with default invocation (see: https://github.com/yi-editor/yi/pull/435.)

Thus it would be nice to have %option pragma that could add any command-line options. (Like GHC does with {-# OPTIONS_GHC #-}.)