Add update mode

This mode allows a user to append to nodes in the YAML file.

This can be useful when programmatically building up a set of configure
commands, for example, instead of resorting to a Python hook file.

Example use:

```
  - when: pyver == '3.4'
    name: configure
    mode: update
    extra: ['--without-ensurepip']

  - when: python_framework
    name: configure
    mode: update
    extra: ['--enable-framework=${ARTIFACT}']
```
@ahmadia can you help me finish the test? I didn't figure out how to access the 'extra' variable to check that it got assembled properly.