Throw error if unquoted attribute value is made up of more than one node

Resolves https://github.com/tildeio/htmlbars/issues/182.

The deleted lines in `token-handlers.js` were redundant and were mostly removed just to clean it up a bit.  But line 89 in particular needed to go because otherwise we wouldn't be able to get the line number with something like `class={{foo}}bar` (by the time we get to adding `bar` we'd no longer be on an attribute with a mustache from which to get the line number). 
Ah my mistake. Didn't read it over carefully enough. The marking an attribute as quoted should only happen between tokenizer state transitions. This is precisely what is happening in `foo={{bar}}` - we are forcing the tokenizer to switch into the unquoted attribute value state, as if it had encountered any other character.