Add 'plugin' support for custom commands and config options.

[tohagan]

Node and cli-config makes plugin support a no-brainer ...

• We might add this into cli-config or implement it as a separate npm package.
• TODO: Review other npm plugin support packages

Implementation Ideas

1. Add plugins property in .stlive.json
   • maps new top level commands to a node package called using cli-config interface.
   • plugins: { "mycmd": "npm-package", ... }
2. Publish plugins as new npm packages.
   • Package names start with stlive- prefix and are registered with the 'stlive' keyword.
3. [sudo] stlive plugins install
   • Creates plugins folder
   • Generates plugins/index.js and plugins/packages.json from config.plugins .
   • Runs npm install in plugins folder
4. [sudo] stlive plugins update regenerates files and runs npm update in plugins folder.
5. stlive [help] [cmd ...] - sets config.help = true and calls command function
   • Command functions should now display usage help when config.help === true.
6. stlive plugins list - Lists configured plugins
7. stlive plugins index - Opens https://www.npmjs.org/browse/keyword/stlive

[tohagan]

It's likely that plugins will want to use peerDependencies