Add support for computed properties in parse_js parser

[iafan]

Newer versions of ECMAScript allows one to use computed properties (key names), e.g.:

{
    [someVar]: 'value',
    [someVar + 'suffix']: 'another value'
}

Technically, there can be any expression within square brackets. It would be good to allow to use such properties for string extraction. A reasonable limitation would be to require the entire expression to stay on a single line.