Add prepend arg to toc extra (#397)

[Crozzers]

This PR closes #397 by adding a flag that enables the TOC to be prepended to the output text.

markdown2.markdown(text, extras={'toc': {'prepend': True}})

This option is off by default to maintain backward compatibility

[nicholasserra]

Don't we do this somewhere else also? Maybe for some table extra. This is giving me deja vu

[Crozzers]

Can't think of anywhere else we do this. The only recent changes to TOC I can think of is with mixed header-ids where we sort the TOC by order of appearance. Only other place that toc is outputted seems to be in the CLI

python-markdown2/lib/markdown2.py

Lines 3313 to 3315 in 749ee72

	if extras and "toc" in extras:
	log.debug("toc_html: " +
	str(html.toc_html.encode(sys.stdout.encoding or "utf-8", 'xmlcharrefreplace')))

[nicholasserra]

Yup, I'm remembering adding support to the CLI output here

#283

[nicholasserra]

Needs a rebase or merge in

[Crozzers]

Rebased onto master

[nicholasserra]

Thanks!