Add flag validators

[yin]

GFlags uses Validators to quickly resolve any ill-formatted flag value with an application crash and a useful message. The validators are defined similarly to defining a flag and the two stay close together.

Consider following syntax:

 private static final Flag<Integer> angle = Flags.integer('angle')
 .validate(new Validator<Integer>() {
 @Override public boolean check(String flagName, Integer value) {
 return value >=0 && value < 360;
 }
 });

Java 8:

 private static final Flag<Integer> angle = Flags.integer('angle')
 .validate((flagName, value) -> value >=0 && value < 360);

[yin]

Done in v0.3.0-beta1

[yin]

Missing tests, reopened