Add support for case insensitive filesystems

[wiwichips]

For example, using NTFS on windows results in this behaviour:

.\\Example.js

exports.hello = 'world';

.\\main.js

...
const hw1 = ctxRequire('.\\example.js');
const hw2 = ctxRequire('.\\EXAMPLE.JS');

hw1.hello = 'Jupiter';

console.log(hw1); // prints { hello: 'Jupiter' }
console.log(hw2); // prints { hello: 'world' } <-- but should be { hello: 'Jupiter' }...

assert.equals(hw1.hello, hw2.hello); // assertion error...

[wiwichips]

This was largely implemented in #5

But this PR should close the issue: #9