Support handlebars block shortcuts

[ahumphreys87]

As well as using if blocks, handlebars also supports using a shortcut for booleans and arrays, this doesnt seem to be documented however.

The following test case illustrates its use:

it('boolean', function() {
    var string = '{{#goodbye}}GOODBYE {{/goodbye}}cruel {{world}}!';
    shouldCompileTo(string, {goodbye: true, world: 'world'}, 'GOODBYE cruel world!',
                    'booleans show the contents when true');

    shouldCompileTo(string, {goodbye: false, world: 'world'}, 'cruel world!',
                    'booleans do not show the contents when false');
  });