Add --number-toc command line arguemnt.
If provided {{toc}} will produce numbers on the left of the titles in the TOC.

Example:

    \# The document
    \#\# headingA
    \#\#\# headingB
    \#\# headingC

Produces:

    The Document
    1 - headingA
    1.1 - headingB
    2 - headingC