Make fluvio update update plugins as well as CLI

[nicholastmosher]

Closes #672

This updates the fluvio update command to behave as follows:

• fluvio update with no parameters will update the CLI and any plugins that support being updated* to the latest stable version
• fluvio update <pluginA> <pluginB> will update any of the named plugins to the latest stable version
• fluvio update with a --develop flag with update the targets to the latest prerelease (rather than stable)

*Plugin Update Mechanism

In order to be compatible with auto-updating, each plugin must now provide a new field of metadata called package, which identifies the plugin on packages.fluvio.io. For example, the fluvio-cloud plugin would provide a package field containing fluvio/fluvio-cloud. This is delivered via the already-existing metadata mechanism, where any plugin executed with a metadata argument is expected to return a FluvioExtensionMetadata, e.g.

$ fluvio-cloud metadata
{
  "title": "Fluvio Cloud CLI",
  "package": "fluvio/fluvio-cloud",
  "description": "A tool for interacting with Fluvio Cloud",
  "version": "0.1.4"
}

In the JSON above, the package field is new, and allows the plugin to tell the CLI where to search for its updates. Older plugins that do not provide the package field will not be auto-updated with fluvio update (with no arguments), but updates will still work fine if the plugin to update is named, e.g. fluvio update fluvio-cloud.

Example:

[simlay]

I think I'm okay with this.