https://github.com/kubernetes-helm/community/blob/master/helm-v3/003-state.md#namespacing-changes

Support installing plugins by relative path

```
helm plugins install .
```

Newer gits don't like checking out an empty string.

```
empty string is not a valid pathspec. please use . instead if you meant to match all paths
```

defer statement executes only when function return, and the resource still be hold during loop. Release the resource manually when not needed.

* fix a number of issues with flag parsing
* add support for `HELM_DEBUG`
* lazy expand flag default envars

Fixes https://github.com/kubernetes/helm/issues/2488

If git has autosetuprebase enabled pulling will fail without a
descriptive error message.

* 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

Partially fixes issues/2385 - helm install silently updates the plugin, if it pre-existed

Fixes issues/2384 - helm plugin install installs a default version in case requested version is not available

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.

Add plugin management subcommands for installing and removing plugins
to `$HELM_HOST/plugins`.

Install accepts a vcs url or a local directory.

```
$ helm plugin install http://github.com/adamreese/helm-env
Installed plugin: env

$ helm plugin list
NAME    	VERSION	DESCRIPTION
env     	0.1.0  	Print out the helm environment.

$ helm plugin remove env
Removed plugin: env
```

closes #1977

In the TestLoadDir test case, currently reflect.DeepEqual is expected to fail by error. Expected metadata is different than in the fixture yaml. Also, the type of the struct is mismatching in DeepEqual call (struct vs pointer to struct).

This adds a backend for a plugin system.

Closes #1572