Use DOM manipulation when replacing HTML elementattributes

[Ocramius]

Originally posted by @Ocramius in #376 (comment)

We should look into getting rid of string replacements, and using DOM manipulation for HTML replacements, where possible.

This is NOT a security issue because this tool is intended for use on repository sources, NOT user input.

[Ocramius]

Mandatory Zalgo: https://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags

[asgrim]

I've started work on this, although it will need a lot more time put into it than initially anticipated, as the resulting HTML is not quite right. Branch with my progress so far is at https://github.com/Roave/DocbookTool/tree/use-dom-manipulation-for-confluence-writer (but note: it is failing, and very much WIP! not ready for PR review yet)