Suppress pkgdown's theme dependencies from examples and article content

[gadenbuie]

This PR resolves problems like the one seen in #1929 where adding htmltools objects (htmlwidgets, etc.) in reference examples or in articles could result in multiple versions of jQuery or multiple Bootstrap JS/CSS files being loaded on the page.

Rather than modifying pkgdown_print(), the approach here to follow the lead from htmltools::suppressDependencies() and to create empty deps for jQuery and Bootstrap with versions 9999 that are either added to the dependencies returned by the examples or provided to the extra_dependencies argument of the html_document() format that is used for articles.

Note that asis articles are left as-is — if required, authors can still use suppressDependencies("jquery", "bootstrap") in the articles source.

Because this is done at a higher level than I initially anticipated, duplicated dependencies are dropped silently. But this matches how dependencies are resolved among htmlwidgets in the same .Rmd file with competing dependencies, so I think that's acceptable behavior.

[hadley]

Would this approach also work for #1798? (And hence reduce the hackiness)

[gadenbuie]

Yeah, I think it would! You'd like to suppress the anchor sections javascript for both Bootstrap versions, correct?

[hadley]

@gadenbuie correct

[hadley]

Thank you!