* clean up plugins GoDoc:
- eliminate plugin.NewBlankContext() as unnecessary
- export ValidIdRegex as a string vs. the less readable var
- add/update various documentation strings
- hide everything by default, except where used by client plugins or the mattermost-server. The exception to this rule are the `*(Args|Returns)` structs which must be public for go-plugin, but are now prefixed with `Z_` with a warning not to use.
- include a top-level example to get plugin authors started
This is not a breaking change for existing plugins compiled against
plugins-v2.
* remove commented out ServeHTTPResponseWriter
* update examples to match developer docs
* add missing plugin/doc.go license header
