feat(helm): add plugin management commands

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