Add htmlStrike syntax highlight group to builtin colorschemes

I noticed that in-spite `htmlStrike` is defined in the bundled `markdown.vim` syntax  file, it is not defined in a number of colorschemes. 

It would be nice to include it. 

EDIT; as workaround I added the following to my `~/.vim/after/ftplugin/markdown.vim`

```
augroup MarkdownStrike
  autocmd!
  autocmd ColorScheme * highlight htmlStrike gui=strikethrough cterm=strikethrough
augroup END
```
