* Adds update option to plugin command
Fixes issues/2385 - helm install silently updates the plugin, if it pre-existed
* Added tests for new methods for plugin update
* Updated docs
* Updated review comments :)
* Return error exit code when there is error
It is now possible to create plugins with chart download capabilities for custom, non-http protocols.
Furthermore it is possible to reuse helm packages to implement alternative clients with these custom downloader functions.