Parse pragma to include and remove sections from READMEs in the docs

[agubler]

Implements rudimentary parsing of two pragmas:

• <!--READMEONLY-->
  • a pragma used a the beginning and end of content that should be excluded when rendering the content for the docs site
• <!--DOCSONLY-- & --DOCSONLY-->
  • a pragma that wraps content that should be excluded from the README but included in the docs site

<!--READMEONLY-->
Section will render on the readme but not on the docs
<!--READMEONLY-->
<!--DOCSONLY--Section will render on dojo.io docs but not on the README--DOCSONLY-->

Resolves: https://github.com/dojo/dojo.io/issues/467