Invalidates access tokens of a given Mojang account.

@param {Object} credentials - account credentials
@param {string} credentials.username - email or username of a Mojang account
@param {string} credentials.password - password for the given account
@returns {Promise} resolves only if credentials were correct
@see {@link http://wiki.vg/Authentication#Signout}
@example
mojang.signout({username, password})
.then(() => console.log('user signed out'))
.catch(err => console.error(err))
function signout ({username, password}) {
  return fetch(`${YGGDRASIL_API}/signout`, {
    method: 'POST',
    body: JSON.stringify({
      username,
      password
    }),
    headers: {
      'user-agent': USER_AGENT,
      'content-type': 'application/json',
      'accept': 'application/json'
    }
  })
    .then(handleErrors)
    .then(res => null)
}