Conversion between html and markdown in Rust with whitespace

[alunturner]

We currently have a slight issue with converting html to markdown in the rust model. Issue is as follows:

• html can support whitespace inside tags eg <em>hello </em> is valid html and if a user were to click italic, type 'hello ' and then click italic again to disable the italic mode, this is the html they would have input
• when we convert this to markdown, the output is _hello _
  • this is not valid markdown due to the space before the final underscore
  • this means that if we try to parse that markdown back into html we get the output _hello _

In summary, converting html => markdown => html at the moment is not perfect due to the difference in how the formats handle whitespace at the end of a 'node'.

[jonnyandrew]

Closing as we don't currently aim to produce machine-readable markdown that can do round-trips to/from HTML (see #800).