Add support of capability api

[jacstn]

1. Objective

Add support of capability API

2. Description of change

• added new resource responsible for capability API call.
• updated index.d.ts
• added automatic tests

3. Quality assurance

Install omise-node library in your project using NPM command npm i git:github.com/omise/omise-node.git#capability-api

retrieve capabilities using the following code example:

const omise = require('omise')({
    publicKey: 'pkey_test_XXXXXX',
    secretKey: 'skey_test_XXXXXX',
    omiseVersion: '2019-05-29'
  });
 omise.capability.retrieve(function(err, capability) {
    console.log(capability);
  });

check if capabilities are properly displayed.

or use auto-tests - run npm test

4. Impact of the change

N/A

5. Priority of change

Normal

6. Additional Notes

N/A