Handle \0 and other characters

[mortenpi]

If people add explicit \0 characters into docstrings (e.g. https://github.com/tpapp/StataDTAFiles.jl/blob/v0.3.0/src/StataDTAFiles.jl#L103), those get passed on all the way to the writers and we just end up with \0 characters in the generated HTML. We probably don't want that, so we should sanitize the user-provided strings before we output them.

Not entirely sure when we should do it. Obvious option is in HTMLWriter, but maybe it should already be done in the Markdown -> MarkdownAST conversion (open question here: how does CommonMark handle \0-type characters, in the spec and parser?).

Unsure if/how this affects the latex output.

[odow]

Same problem in LaTeX: #1467