Added funtionality to change template delimiters in the template engine.

https://github.com/yeoman/generator-generator/issues/45 and https://github.com/yeoman/yeoman/issues/1235 talk about ways to pass options to lodash to change the template delimiters. However the template engine in yeoman does some detecting and has the `%%` escaping built in. If I try to template a file that doesn't have any `<%``%>` tags in the file it won't get templated.

This pull request adds the ability to provide options to the engine to customize the detection and escaping. For example the following configuration could be used to escape with `{{``}}` delimiters and escape a `{{{` to `{{`.

```
engine.options = {
          detecter: /\{\{?[^\}]+\}\}/,
          matcher: /\{\{\{([^\}]+)\}\}/g,
          start: '{{',
          end: '}}'
        }
```

I also added tests that should cover this added configuration and would illustrate the issue without these changes.