Fix text highlighter

[davekinkead]

Hi Geoff,

This feature should do the following:

• when embedded, offer medium/theguardian style text highlighter
• allow a developer to specify what icons & labels text can be highlighted with
• allow a user to unselect/remove highlighted text
• save highlighted text in local storage (and eventually backend DB)
• load highlighted text from local storage when a user visits that URL
• post highlighted text to dev specified URL (default to POST reasons.io/

Happy to change any code but would like the API in lib/reasons.js to remain the same if possible