Add differential output option

[liangdrew]

When enabled, only fields that have changed between the previous and current state will be displayed when using step or alt.

(aldb) help set
Set ALDB options.

Available options:

set conf [filename]

    Set the parsing configuration for the current session.

    The specified file must be in YAML. The following (customizable) properties are set by default:

    # Name of the sig representing the main state in the Alloy model.
    stateSigName: State
    # Name of the predicate which defines the initial state in the Alloy model.
    initPredicateName: init
    # Name of the transition relation in the Alloy model.
    transitionRelationName: next
    # Additional Alloy sig scopes to specify.
    additionalSigScopes: {}

    Running set conf with no filename will set the above default options.

set diff <on | off>

    Turn on/off differential output mode.

    When enabled, only fields that have changed between the previous and current state are displayed when executing step or alt.
    By default, this option is enabled.
(aldb)

Closes #39.