Add CLI !config command

[ryannedolan]

Added new CLI command !config, enabling users to dynamically tweak configuration properties. These are then visible within templates, e.g. as rendered by the !yaml command.

Added a new constructor to HoptimatorCliApp, enabling client code to provide custom configuration.

Testing Done

Rendering YAML with the !yaml command may result in unexpanded variables, e.g. {{pipeline.namespace}}:

> !yaml insert into ...
--->%---
  namespace: {{pipeline.namespace}}
--->%---

With the new !config command, we can specify these variables dynamically:

> !config pipeline.namespace=testing-123
> !yaml insert into ...
--->%---
  namespace: testing-123
--->%---