allow rendering directly from template strings

The need to use the file system is pretty much useless in an advanced
system with its own file loading mechanisms with support for file
system mocking. That is why it is beneficial to have this simpler
method for rendering.

```
var template = ‘html string with <%- @ect_code %>’;
var data = { ‘ect_code’: ‘what up’ };
var rendering = ECT.renderFromString(template, data);
```