Add vimdoc @tag directive to render arbitrary help file tags

Vimdoc supports auto-generated tags for most documentation items (sections, functions, commands, etc.), but doesn't have a way to render arbitrary tags.

For instance, something like this would be useful for the maktaba docs:

```
" @subsection Default Flags
" @tag plugin[]
" @tag plugin[commands]
" @tag plugin[settings]
" @tag plugin[autocmds]
" @tag plugin[mappings]
" @tag instant[]
" Maktaba provides a few built-in flags in every plugin that…
```

It might also be worth having a corresponding `@link(TAG)` notation for links instead of using literal helpfile link syntax, "|TAG|", so it works better for generated web documentation. But it would also be possible to process the pipe syntax explicitly and render appropriate web links in web documentation, so maybe we don't need a special directive for it.

We could cross that bridge when we come to it, of course. Or when we get around to open-sourcing vimdoc, at least.