read stdin/write stdout

[fbicknel]

It might be very helpful if toc-md could read stdin and write the result to stdout. This would make filtering a file in vim produce the TOC.

Seems safe (and desired) if you made this the default, but a command line argument would be helpful if you can't.

Example use (in vim):

:1,$!toc-md<return>

Or, from normal mode:

gg!Gtoc-md<return>

Thanks for reading!