Add a field widget for textareas to set the width

[briancwald]

Textareas are now set to 100 cols per the ticket on material_admin briancwald/material_admin#35

As an addition, It would be nice to specify an override width on a per textarea basis. We should add a field widget that provides 2 things:

1. a numeric value for column width which will update the <textarea cols="HERE">
2. an option (checkbox) to create a fullsize text area,
   reference: suggestion : bigger text-area for the content field  contentacms/contenta_jsonapi#178 (comment)