YAML filter: handle "plain flow" style multi-ling values

[Chase Tingley]

Follow up to issue #643.

From this rundown of different methods for multiline scalar values:

plain style (no escaping, no # or : combinations, limits on first character):

Key: this is my very very very 
  long string

The value can even start on the next line, like this:

key: 
   here is a string
   across multiple lines

Although we currently handle other multiline string cases, we don't handle this one.

[Chase Tingley]

Updating the title; this is an issue with the YAML filter, not the markdown filter. Oops!

[Chase Tingley]

Reference to the YAML spec: http://www.yaml.org/spec/1.2/spec.html#id2788859