Make command line option headings linkable

[victorlin]

This will be useful for pages such as Filtering and Subsampling which reference augur filter command line options.

Proposed in #1431 (comment).

@tsibley said:

The individual command-line options for augur filter can be made linkable, for sure. There's Sphinx support for it, and I've done it for Nextstrain CLI command reference pages. Implementation may be a little different for Augur since that's generating the reference pages another way.