Added options parameter to renderers.

To keep the API simple and avoid API changes when new options are
added, this is just a long integer.

Set it by disjoining options that are defined as powers of 2:  e.g.
`CMARK_HTML_SOURCEPOS | CMARK_HTML_HARDREAKS`.

Test options using `&`: `if (options & CMARK_HTML_SOURCEPOS)`.

Added `--hardbreaks` and `--sourcepos` command-line options.