* The `--strict` option has been removed.
* Instead of using `--strict`, one can now use `strict` instead of
`markdown` as an input or output format name.
* The `--enable` and `--disable` optinos have been removed.
* It is now possible to enable or disable specific extensions
by appending them (with '+' or '-') to the writer or reader
name. For example `pandoc -f markdown-footnotes+hard_line_breaks`.
* The lhs extensions are now implemented this way, too; you can
use either `+lhs` or `+literate_haskell`.