Add yaml support to init container

[yaron-idan]

Generating yaml configurations is becoming popular, in my case I found the need for it because that is the format rtmbot is expecting.

Adding support for this in Kamus can ease onboarding for many services, so I'll open a PR for it.
WDYT?

[yaron-idan]

After some work, it seems supporting yaml for secrets encryption could miss some of the advantages of the format, since kubernetes configmaps cannot handle arrays or nested objects.
A better approach might be allowing users to supply Kamus with a template file.
This feature should include -

• Adding a template option to the kamus cli
• Setting a clear standard for kamus secret placeholders in the template
• Allowing the init container to inject secrets into the template
  WDYT @omerlh ?

[omerlh]

Sounds interesting - maybe as a refactoring of the current init container, with JSON as a template and allowing user-supplied template.