Add support for read-only fields

[darinwilson]

There are cases where fields should be visible to admins, but not editable, e.g. timestamps. The user could specify this on the command line by appending :readonly when generating the admin area on the command line:

$ mix torch.gen eex Admin Post posts title:string body:text inserted_at:date:readonly

The generated template could either put the value in a disabled input, or just display it as plain text. The controller would exclude read-only fields when creating changesets.