Custom validation for built-in rules

[erfanium]

Related to #100
Customizing built-in rules, without creating new rule.
For example:

check = v.compile({
   num: {
      type: "number",
      min: 10,
      max: 15,
      integer: true,
      custom (value, schema) {
         if (value % 2 !== 0) return [{ type: "evenNumber", actual: value }];
      }
   }
});

[coveralls]

Coverage remained the same at 100.0% when pulling 384c3b6 on erfanium:master into d1edb33 on icebob:master.

[erfanium]

@icebob
Any opinion? Proposal? Criticism? :-)

[icebob]

The idea is good.
But I should check the code & check the performance of PR. If it's ok I will merge it.