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).