Post Attachments (images, video files)

[ChaseC99]

Users should be able to add attachments (like images) to a message.

• Add a file selector to the MessageInput component.
• In SlackClient's postMessage method, add an optional param for file.
  If the file exists, instead of .chat.post, we'll need to use the .files.upload function with the following form data:
  { file: {filepath}, channels: {channel}, initial_comment: {message} }