Adding 'filter' helper for collections

Adding a 'filter' helper for collections that can filter on
strings or object property/value:

```handlebars
{{#filter collection "SearchValue"}}
  <!-- render something for each value that comes back -->
  {{this}}
{{else}}
  Nothing returned
{{/filter}}
```

```handlebars
{{#filter collection "SearchValue" property="SearchProperty"}}
  <!-- render something for each value that comes back -->
  {{this.SearchProperty}}
{{else}}
  Nothing returned
{{/filter}}
```
Merge pull request #84 from doowb/master

Adding 'filter' helper for collections