switch to calver; add g:vimwiki_version; tidy version hardcoding

Setting a global for g:vimwiki_version doesn't seem too egregious, and
lets us reference it cleanly elsewhere.

Removes version header from vimwiki.txt and hardcoding in tagfile
generation.

There's still one instance of that left in test/tag.vader that we should
clean up eventually.

autoload/vimwiki/tags.vim

@@ -292,9 +292,8 @@ function! s:write_tags_metadata(metadata) abort
     endfor
   endfor
   call sort(tags, 's:tags_entry_cmp')
-  " TODO: VimWiki version probably shouldn't be hardcoded here:
   let tag_comments = [
-    \ "!_TAG_PROGRAM_VERSION\t2.6.0",
+    \ "!_TAG_PROGRAM_VERSION\t" . g:vimwiki_version,
     \ "!_TAG_PROGRAM_URL\thttps://github.com/vimwiki/vimwiki",
     \ "!_TAG_PROGRAM_NAME\tVimwiki Tags",
     \ "!_TAG_PROGRAM_AUTHOR\tVimwiki",