Allow accessing the globals with index without a preceding identifier

[imarkkinen]

Fixes #543

let parser = liquid::ParserBuilder::with_stdlib().build().unwrap();
let tpl = parser.parse("{{['other']}}").unwrap();
let globals = liquid::object!({
        "my": {"variable": "value"}, "other": "other_value"
     });
 let res = tpl.render(&globals).unwrap();
// "other_value"

Currently the above fails as it's not considered valid liquid markup in liquid-rust.

• Tests created for any new feature or regression tests for bugfixes.