Add support for iface type

[synaptiko]

Overview

Summary

Loosely based on #116
We use iface in our project, it's really similar to shape, the difference is that only specified props are checked.

Here's an example:

propTypes: {
  dog: PropTypes.iface({
    bark: PropTypes.function.isRequired,
    color: PropTypes.string
  }).isRequired
},

Then you can pass:

dog: Ember.Object.extend({
  bark() {
    console.log('woof woof')
  }
}).create()

or

dog: Ember.Object.extend({
  bark() {
    console.log('ruff ruff')
  },
  color: 'black',
  breed: 'affenpinscher'
}).create()

Both are accepted.

Checklist

If there will be interest in this PR, I can go over the list below and update documentation and provide some tests.

• I have added tests that prove my fix is effective or that my feature works
• I have evaluated if the README.md documentation needs to be updated
• I have evaluated if the /tests/dummy/ app needs to be modified
• I have evaluated if DocBlock headers needed to be added or updated
• I have verified that lint and tests pass locally with my changes

Semver

This project uses semver, please check the scope of this PR:

• #none#
• #patch#
• #minor#
• #major#

CHANGELOG

Support for iface type which is checking provided props similar to shape type but the missing props are tolerated.