Add `summary_opts()` for Markdown summary rendering options
Add `summary_opts()` for Markdown summary rendering options

We have a similar function `opts()` that is for rendering the main body
of the documentation, but until now we just constructed the options for
rendering summaries on the fly. This is a problem if/when we change the
enabled options since the different places can get out-of-sync.

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust