Continue on fail isn't handled at the Stage level

[cmmorrow]

Describe the bug
The --continue option can be provided on the command-line to apply continue on fail behavior to all stages. However, individual stages can not be set to continue on fail, despite continue on fail: true being available for each stage.

To Reproduce
Steps to reproduce the behavior:

1. In a new Config File, create a stage with continue on fail: true
2. Create a command that will deliberately fail such as cp.
3. Run build-magic with the Config File.

Expected behavior
continue on fail: true should work per stage.

Additional context
The problem is because continue_on_fail is an argument for the Engine constructor and not the Stage.build() method. The continue on fail behavior should be moved to the Stage object.