Add new option g:emmet_docroot to specify map of document root.

let emmet_docroot = {
\  'c:/dev/vim-jp.github.com/': 'http://vim-jp.org/',
\  'c:/dev/mattn/': 'http://mattn.kaoriya.net/',
\}

When getting image size of '/path/to/file.jpg' and current html file is
'c:/dev/mattn/foo/bar/index.html', the image file will be treated as:

  http://mattn.kaoriya.net/path/to/file.jpg