Add keywords option

[franciscomorais]

Add the keywords option to the Ajv options to add custom keywords, without having to call addKeyword.

What version of Ajv you are you using?
6.10.2

What problem do you want to solve?
Add keywords object directly to the Ajv constructor, like formats.

new Ajv({
  keywords: {
    constant: {
      validate: function (schema, data) {
        return true;
      }
    }
  }
});

What do you think is the correct solution to problem?
Copy the implementation of the formats option to add keywords.

Will you be able to implement it?
yes

[epoberezkin]

merged