Implement: action/command grouping and chaining

This is to implement the output design from #108

Reasons

[x] Create yml files for all tools in pkg/custom/source.
[x] Create default groups.yml and add it to pkg/custom/source
[x] Document which variables are available for tool authors.
[x] Design groups.yml syntax.
[x] Integrate groups with command line.
[x] Add validation between group name and custom command name.
[x] Modify usage (--help) to show groups in a separate area as compared to top level build in actions.
[x] Order of actions in the group should be the order of execution.
[x] Support running init with no backend (for ci tools that need init)
[x] Refactor custom actions to be a single yaml file instead of multiple.
[x] #129