Create "Experimental" plugins directory & builds

[sparrc]

We need to create a system where new plugins are typically only merged into an "experimental" directory.

This experimental directory would not be a part of the main telegraf build, but we would make builds that include the experimental directory.

The plugins therein would have no guarantees on their consistency across versions until they are merged into the "main" repo.

[sparrc]

#1717 closing this in favor of "external" plugins