Add prop onInputTextChanged on GiftedChat API and help fix #286

[jayshah123]

This introduces a new prop : onInputTextChanged on GiftedChat API,
which is a callback function that will give whatever text is present currently in Composer back to user.

• Composer has onChange prop but it is kind of consumed by onType function in GiftedChat.js.
  This will also help fix Strange behavior with onChange in overridden Composer for typing indicator for that user #286, where you see GiftedChat.js personally uses onChange,
  and the behaviour such as height calculation would have to be performed by library user (which right now happens in onType in GiftedChat.js)

• This prop gives users a simpler interface to track change in input text, with all other features intact.

Kindly guide with suggestions/comments wherever feels necessary.
Thank you.

[morelazers]

Can we get this one merged? @kfiroo

[ghost]

How can we get the text onInputTextChanged?