Handle a new `.tpl.jade` file extension

Putting each template in its own separate file and naming the file
after the template it contains is becoming a followed pattern among
Meteor developers. See for instance this article from Josh Owens:
http://joshowens.me/how-to-organize-your-meteor-js-app/

But as it stands today, this pattern doesn't respect the “don't
repeat yourself” (DRY) philosophy. Indeed you have to wrap your
template in a `<template name="myTemplate>" tag and saving it in a
`myTemplate.html` file, effectively writing the name of the template
twice. If those two names doesn't match Meteor will consider the name
of the `<template>` tag and will ignore the file name. So if you
follow this pattern you have to take care of keeping the file name and
the template tag name in sync (manually).

This commit solve the problem presented above for jade using a new
file extension, namely `.tpl.jade`. With this extension you can only
define one template per file and you don't need to wrap your template
in a tag. The template will be named after the file name. We handle
special `head.tpl.jade` and `body.tpl.jade` templates as expected.

Of course the `.jade` extension is still here and you can use it along
the new one, for instance for rapid prototyping (with several
templates in the same file) or for different parts of the apps (
sometimes it makes sense to define a group of templates in the same
file).