Plugins should be able to access files in their dir

[pauldix]

For some plugins, they may have configuration, HTML, or other asset files that they want to access during plugin execution. They should be able to access files in their directory. For plugins loaded from GH, it should be updated to pull down all the plugin files into the local server directory so they will be available.

Update the functionality and update the documentation in https://github.com/influxdata/docs-v2