feat: support assignments and promise syntax

[thomaslombart]

This PR intends to support the edge cases of the queries usage. That is to say:

• Assignments of the queries :

const promise = findByText('foo')
return promise

• Promise syntax

const promise = findByText('foo')
promise.then(node => done())

// Or

findByText('foo').then(node => {
  done()
})

All tests passes but I consider the implementation to be a bit fragile.

Closes #1
Closes #2

[Belco90]

Awesome! I'm learning lot of things about AST with this plugin :)