Expand tests to improve code coverage

[JeanMertz]

There's currently a single test, to validate the render function:

bulletin/src/main.rs

Lines 116 to 132 in e16f06e

	#[cfg(test)]
	mod tests {
	use super::*;
	#[test]
	fn test_render() {
	let template = "{{ posts[0].href }}";
	let mut posts = vec![Post::default()];
	posts[0].href = "foo".to_owned();
	let data = Data { posts };
	let result = render(template, data);
	assert!(result.is_ok());
	assert_eq!(result.unwrap(), "foo".to_owned());
	}
	}

It would be nice if we could expand this to cover some more functionality, even though I realise there's not much to the complexity of this library, it can still be a nice learning experience to add more tests to the project.