Allow configuration of display text within content_tag

[SirRawlins]

At the moment the value option is used to generate the content within the content_tag output, however, I find myself often wanting to display cropped or truncated content i.e.

<span class="editable" value="Once upon a time in the west.">Once upon a time....</span>

I'd like to propose either a new option to the edtiable() helper to allow configuration of this:

<%= editable @book, :title, display: @book.title.truncate(10) %>

Or perhaps allow the passing of a block which is yielded within the content_tag.

<%= editable @book, :title do %>
    <%= @book.title.truncate(10) %>
<% end %>

Anyone have thoughts on this? I'd be happy to look at putting a PR together unless there are objections to this kind of behaviour in the helper?