upload button support

[huobazi]

I like a upload button can browse local image or use camera on mobile devise

[jeremy]

→ added to Roadmap Questions list!

[tanin47]

I've built one. Please see: https://github.com/tanin47/trix/pull/1/files

Here's how it works:

1. The upload button is added to the toolbar. When clicked, it fires an event with triggerEvent 'click', onElement: @element.querySelector(fileSelector)
2. The file dialog is open. The user can select a file.
3. After a file is selected, element.editor.insertFile(file) is invoked.

Since we use element.editor.insertFile(file), you can use http://trix-editor.org/js/attachments.js to handle the uploaded file.