Update docs and examples to use the draft-js v0.10 API

[gmcnaughton]

Examples and documentation still use the draft-js v0.9 API for creating entities (Entity.create and Entity.get).

We should migrate these examples to the v0.10 API as we move apps forward to the latest version!

• https://github.com/HubSpot/draft-extend/blob/master/building-plugins.md
• https://github.com/HubSpot/draft-extend/blob/master/example/link.html#L55
• https://github.com/HubSpot/draft-extend/blob/master/example/token.html#L81
• https://github.com/HubSpot/draft-extend/blob/master/example/mention.html#L99
• https://github.com/HubSpot/draft-extend/blob/master/src/plugins/utils.js#L38