#41 Use tags file to allow creating links

[fritzophrenic]

Cool idea: https://github.com/vim/vim/issues/7745
We would use the tags file (default to first found tags file in 'tags' option, or else parse all of them, or else specify a file in a new TOhtml option). Process the text to find any tags listed by name in the tags file. Need some way to deal with multiple tags matching at any given location. Then add links to the document for all those found tags. Intra-document links would be easy, will need to specify base path or directory for links to other documents. Should be off by default.
Could also be used to implement: https://github.com/vim/vim/issues/2480 (tags file can contain entries for file names).