Add tag completion

[anjiro]

It’s got a bit of a hack to avoid matching the end-of-tag marker when in findstart mode, but it seems to work in my basic testing. It does depend on g:wiki_tags_format_pattern having a \ze, but since that’s in the default pattern and is featured in the documentation examples, it seems likely to be fine.

[lervag]

Thanks! The code looks good. It would be nice if you could also add:

• Some documentation (in doc/wiki.txt in the completion section).
• Either one or more simple tests (see test/test-complete/test-wikilinks.vim for an example), or as a minimum some example files in test/wiki-basic or similar, which I can use to add the tests myself.

[anjiro]

Added docs, will look at tests in the next few days.

[anjiro]

I added some extra tags to to tagged.wiki to enable basic testing. I’m doing almost all of my work with iVim on iOS at the moment so implementing a full test is a bit too complex.

[lervag]

Thanks! I've added test and made some minor modifications.