improve multi validate

[0x0a0d]

in multi check process, the value passed throw validate function that can changes the original value (type 'object' with strict 'remove')
example:

const Validator = require("fastest-validator");

const v = new Validator();

v.alias("targetA", {
  type: "object", strict: "remove", props: {
    a: "string"
  }
});

v.alias("targetB", {
  type: "object", strict: "remove", props: {
    b: "string"
  }
});

const schema = {
  tester: ["targetA", "targetB"]
};

v.validate({ tester: {a: "ok"} }, schema);
v.validate({ tester: {b: "not-ok"} }, schema);

this pull includes

• add clone, removeChildrenAndMerge method to validator
• add context.validator that ref to validator instance

[coveralls]

Coverage decreased (-0.2%) to 99.404% when pulling 6a28075 on 0x0a0d:master into 34ebcb4 on icebob:master.

[coveralls]

Coverage increased (+0.01%) to 99.602% when pulling 099a4f3 on 0x0a0d:master into 34ebcb4 on icebob:master.

[icebob]

Please add a test which covers the original problem