Determine terminal window height automatically on Unix

As discussed in #14, this patch allows `ghcid` to determine the terminal window height for output formatting automatically when passed `-h0` on the commandline. Uses the [terminal-size](https://hackage.haskell.org/package/terminal-size) package and currently only works on POSIX systems.

The patch dispatches the implementation using CPP in `Platform.hs`; if desired, it could be reworked to use one module per platform and do the dispatch in the Cabal file. There's also still missing the Cabal logic for not depending on `terminal-size` on Windows.

Comments welcome!