Add wrappers to compile frontend outside of GHC source tree
Inject `--interactive`/`-B` options & modify GHCi banner
backport readMaybe
add ":complete" command exposing repl completion

This is a proposed feature for GHC 7.8.1
see also

  http://ghc.haskell.org/trac/ghc/ticket/5687

Note, this commit corresponds to

  http://ghc.haskell.org/trac/ghc/attachment/ticket/5687/ticket-5687-ghci-complete.v2.diff