The App module provides a function that does a pandoc conversion,
based on option settings. The program (pandoc.hs) now does nothing
more than parse options and pass them to this function, which can
easily be used by other applications (e.g. a GUI wrapper).
The Opt structure has been further simplified.
API changes:
* New exposed module Text.Pandoc.App
* Text.Pandoc.Highlighting has been exposed.
* highlightingStyles has been moved to Text.Pandoc.Highlighting.
