Add shortcut for alias in specs

[j4mie]

This introduces a new bit of syntax into a spec, providing a shortcut to alias the output of a projector. This brings things a little closer to the syntax used by SSM plugins, where the output key name is clearly visible in the spec.

This allows you to do things like this:

def project_full_name(instance):
    return {"full_name": f"{instance.first_name} {instance.last_name}"}

full_name = (qs.include_fields("first_name", "last_name"), project_full_name)

spec = [
    "name",
    {"whole_entire_name": full_name},
    {"age_in_years": "years_since_birth"},
]

[pmg103]

Something odd about needing to specify the key even though you intend to alias it but this is a sensible PR on its own terms

[j4mie]

Superseded by #43