feat(*): add custom action support

[vdice]

• adds custom action support (porter invoke --action <action>)
• removes status command now that it is superseded by added custom action support
• adds a basic example demonstrating use of custom actions
• adds/updates mixin schema and adds validation tests

Closes #14

[carolynvs]

This looks good. One more thing that you'll want is VSCode support so that you get autocomplete for custom actions.

Here's what that looks like in the exec mixins:

https://github.com/deislabs/porter/blob/0652b3af4c19c63e695ced1b50c6f258f40fd71e/pkg/exec/schema/exec.json#L158-L165

It needs a patternProperty and a definition named invokeStep.

[vdice]

Thanks, @carolynvs !

Finally got around to updating this mixin's schema (including the use of patternProperty for the invokeStep, thank you for the reference) and adding tests. Please see 77cf95d

[carolynvs]