Add a .tpl.coffee format so events and helpers can live together in one file

[tdamsma]

I added a .tpl.coffee format that allows all coffeescript for a particular template to live together in one file like so:

hello.tpl.coffee:

events
  'click button': () -> 
    Session.set 'counter', Session.get('counter') + 1

helpers
  counter: () ->
    return Session.get('counter');