Port some improved handling of multiline string blocks

* Allows keys with decimals to be matched
* Allows proper syntax handling of `#` within a multiline block
* Allows for ending with an indented comment

For more information see https://github.com/atom/language-yaml/pull/19 (it has pictures!) and https://github.com/atom/language-yaml/commit/6e31ad19f18bc488184c7299ba8e9e9af95cb461.