Improve executables (mainly option parsing)

This adds option parsing to both executables, as well as some other [interesting features](https://github.com/hoedown/hoedown/issues/20#issuecomment-36689014). Finished version of my `options_parsing` branch. Things to note:

 * Time measuring `-T` has been commented out, as we need some `configure` logic to handle it properly around platforms. (aka #61)
 * Descriptions have been made clearer.
 * One extension has been added to its correct category.
 * `examples/` is now `bin/`.
 * Common logic between both executables has been moved to `common.h`.
 * `smartypants` now parses options just as `hoedown`.