Added Text.Pandoc.Process (pipeProcess).

A souped up version of readProcessWithErrorCode that uses lazy bytestrings
and allows setting environment.
Use pipeProcess in Text.Pandoc.PDF.
Pass writename as argument to filters.

This way filters can figure out what the target format is
and react appropriately.

Example:

    #!/usr/bin/env runghc
    import Text.Pandoc.JSON
    import Data.Char

    main = toJSONFilter cap
      where cap (Just "html") (Str xs) = Str $ map toUpper xs
            cap _ x = x

This capitalizes text only for html output.