Fix escaping of dollar symbols in HTMLWriter

[odow]

Previously, Javascript LaTeX engines got confused trying to
distinguish between inline delimiters and dollar symbols for actual
currency. This works around the problem by wrapping (currency) dollar
symbols in a span tag.

Closes #890

I could use come pointers on how to test this properly. (Is there a way to test the output of the rendered HTML?) I got confused with the structure of /test. Here's my more-involved local test:

shell> cat src/index.md
The price is \$1.

The price is \$1 or \$2.

Here is some $x^2$ math.

The pr\$ice is \$s for some $x^2$.

julia> Documenter.makedocs(sitename = "Bug", pages = ["index.md"])
[ Info: SetupBuildDirectory: setting up build directory.
[ Info: Doctest: running doctests.
[ Info: ExpandTemplates: expanding markdown templates.
[ Info: CrossReferences: building cross-references.
[ Info: CheckDocument: running document checks.
[ Info: Populate: populating indices.
[ Info: RenderDocument: rendering document.
[ Info: HTMLWriter: rendering HTML pages.

[mortenpi]

LGTM, I think this is a clever workaround. And doesn't have the potentially breaking aspect of changing the delimiter configuration.

We don't really have good integration tests of the outputs. The best we do currently are the occursins in the example builds. Potentially you could add a few tests there -- I think having your snippet in the test/examples build would be good.

Just a passing thought, beyond the scope of this PR, but in the future we could use Gumbo to parse the generated HTML and then do some sanity checks on the AST. It worked well for generating the outdated version warnings.

[odow]

@mortenpi how's this?

[mortenpi]

Looks good @odow, thanks!