[yaml] multiline strings improperly highlighted

[mpnally]

• VSCode Version: 1.1.0
• OS Version: OSX 10.10.5

Steps to Reproduce:
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swagger: '2.0'
info:
  description: 'The API Management Service API defines an updated and refined version
    of the concepts currently known as Developer, APP, and API Product in Edge. Of
    note is the introduction of the API concept, missing previously from Edge

    '
  title: API Management Service API
  version: initial