2.4.0 stopped installing development versions by default. This allows
the user to re-enable that support by setting --devel.

Closes #2407

* 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

helps with half of #2224

Updated the 'helm completion' command to take in the shell
as a parameter. Currently acceptable options are 'bash' and
'zsh'. Also fixed the completions to work with zsh in a manner
similar to what kubectl does.

Also updated the docs to reflect this change.

Closes #2201

This adds a hidden 'helm doc' command that generates documentation out
of the Helm help text.

Currently it can create:

- man pages for HELM(1)
- Markdown documentation
- bash autocompletions

Those are the three built-ins for Cobra.

The command is hidden so that it doesn't show up to the regular user,
since this is really a specialized task. It has the advantage of making
it trivially easy to install man pages and bash completion at installation time.