add indentation support

Add a simple indentation function that looks at nothing but opening and
closes braces.

Ideally we should use `smie' to do indentation, but this works for now.

Given the following file:

    / {
    cpus {
    cpu-map {
    cluster1 {
    core0 {
    cpu = <&CPU0>;
    };
    core1 {
    cpu = <&CPU1>;
    };
    };
    };
    };

    widget: pizza,feast@a00000 {
    compatible = "pizza,feast-v1";
    things = <1 2 3>;
    };
    };

With this change we get the following indentation:

    / {
        cpus {
            cpu-map {
                cluster1 {
                    core0 {
                        cpu = <&CPU0>;
                    };
                    core1 {
                        cpu = <&CPU1>;
                    };
                };
            };
        };

        widget: pizza,feast@a00000 {
            compatible = "pizza,feast-v1";
            things = <1 2 3>;
        };
    };