Allow user to specify link display text by subclassing `UriCell`

Currently a user needs to write a custom Cell class in order to format
URLs with an alternate display text. This requires implementing the
render method and thus duplicating logic in `UriCell`. This commit
allows display text to be specified by subclassing `UriCell` and
implementing a single function `displayText`. E.g. in coffeescript:

class MyUriCell extends Backgrid.UriCell
    displayText: -> @model.get('my_field_name')