runt! html.vim may break other behavior

It is not safe to source html.vim for other file types.

For example,
In my ftplugin/ I have html.vim and xml.vim
html.vim:
`map a b`
xml.vim:
`map a c`
Without MatchTag, I can get right mapping (a->c) in xml, however with MatchTag, I get a->b for xml files, because html.vim is loaded after xml.vim.

We may have 2 solutions:
1. use `so :p:h/html.vim` instead of `runtime` in other file types.
2. wrap everything in html.vim into a function and define `autocmd` in plugin/.

Thanks.