Add argument to number headings in table of contents

Added `--number-toc` as an argument.

Normally `{{toc}}` will produce a list of headings without any indication as to their rank. By providing this argument the generated Table of Contents will include numerical hierarchical descriptors on the left of each heading in the Table of Contents only.

Example:
	# TitleA
	## HeadingA
	### HeadingB
	## HeadingC
	## HeadingD
	# TitleB
	## HeadingE
	### HeadingF

Produces:
	TitleA
	1 - HeadingA