Add support for custom FormData

[safrazik]

As discused in: #5

Proposed API:

// Upload with custom FormData
this.$refs.vueFileAgent.upload(url, headers, filesData, function createFormData(fileData){
  var formData = new FormData();
  formData.append('action', 'upload');
  formData.append('my-file', fileData.file); // this is important! - the actual file for upload
  return formData; // edit: fixed. Thanks to Chauhan-Nitesh for the comment below
});

[Chauhan-Nitesh]

@safrazik First of all I want to thanks for such a useful plugin.

You missed the return statement in code.
return formData;

Otherwise the post data will empty while uploading the file.

[safrazik]

Thanks for the appreciation and thanks for pointing out the issue . Fixed the example. The library code is already fine with it.

if(typeof createFormData == 'function'){
  formData = createFormData(fileData);
}
else {
  formData = new FormData();
  formData.append('file', fileData.file);
}

[abelnexus]

formData.append('id', this.id);
why add dinamic id error undefined 'id'