Removes omit_attr argument from s:register_language(), because html,
javascript, and css regions were the only ones that used it, and they
are now handled by syntax/html.vim.
`runtime! syntax/html.vim` also has the advantage that we can hook into
the html syntax, meaning that we can add stuff like highlighting
javascript expressions inside vue directives, and add our own html
attributes.
Also removes the start of line limitation introduced in previous commit
