improve: add express option for template engine

In init procedure, add code that setup express custom template
engine and an option variable named "custom engine" that permit to
setup a custom template engine.
example:
  engine = require('ejs-locals');
  keystone.init({
      ...
      'custom engine': engine,
      ...
      });
Merge pull request #92 from JeremyFouriaux/master

improve: add express option for template engine