Introduce lightweight frontend indicator for sites using Stream

[frankiejarrett]

Similar to other plugins, such as WordPress SEO by Yoast and Easy Digital Downloads, this will just be a lightweight and invisible indicator (HTML comment) in the page source that will show if a site has Stream activated and what version they are using.

This indicator will aid us in a number of ways, including helping to resolve support requests faster for any users experiencing problems, as well as revealing if the site is even using Stream to begin with.

However, this output should also be filterable, so that if for whatever reason a site owner does not want this appearing in their page source, they can easily remove it.

[lukecarbis]

Solved in #509 .