Accept strings for images on the feature methods.

[lukesneeringer]

We should accept strings on the single-feature methods and build the image object.

Example:

vision.faceDetection('https://location.com/image.jpeg').then(response => ...);
vision.faceDetection('local_file.jpeg').then(response => ...);

We can essentially detect the string and build the right object:

let request = {
  image: {
    correctKey: str,
  },
  features: ['FACE_DETECTION'],
}

Users needing additional arguments (e.g. imageContext) will have to pass the full request object instead.