Add cp932(Windows-31J) support to tidy.vim.

Tidy puts error when editing html file that encoded Shift JIS.
I added cp932 support to tidy.vim on html and xhtml.
cp932(Windows-31J) is Japanese vim user's Shift JIS. 
Further details about Windows-31J is [here](http://www.iana.org/assignments/character-sets/character-sets.xml)