Image attachments support

[fdecampredon]

Purpose

This pull request enhance the plugin with partial support for image attachment.
To add image to sent sms the attachments property of the options should contains an object which has for key the filename of the attached images and for value a base64 string representation of the image, example:

const options = {
  attachments: {
   'image1.jpg' : "....base64string...", 
   'image2.jpg' : "....base64string...", 
  }
}
sms.send(number, message, options, success, error);

IOS Support

The new feature works fine on IOS (at least based on my tests).

Android Support

The plugin only support the INTENT method for image attachement.
Also providing a non empty phone number and attaching multiple image does not work.
I can't understand why but Intent.ACTION_SEND supports providing sms number while Intent.ACTION_SEND_MULTIPLE does not (Intent.ACTION_SEND_TO does not support attachment ...).

Windows

Not implemented

[noriellecruz]

is this something like mms?