Allow to define multiple variables and extract them respectively

[stalniy]

Example:

describe('Suite', function() {
  const { fullName, firstName, lastName } = def.vars({
      firstName: 'BDD',

      lastName: 'Lazy variables',

      fullName() {
        return `${firstName} ${lastName}`;
      }
  });

  it('computes variables', function() {
    expect(fullName).to.equal('BDD Lazy variables');
  });
})

This way library won't pollute global scope with $varName getters.

This is a breaking change as I'm going to increase supported node version to node 4.x (or 6.x) and remove ui for global variables and getters on get function.

[stalniy]

tested, and looks like invalid. Getters are called at the time of destructuring...