Support union type in `Type::nullable()`

[GromNaN]

• new feature: fix #139
• BC break? no
• doc: in readme.md

I did some benchmarks. in_array vs preg_replace approaches are almost identical in terms of performance (very fast). Got the simplest implementation with explode, in_array and array_diff.

[dg]

Thanks

[dg]

I've tweaked it a bit, to make it work case insensitive, because theoretically you can write Null or NULL in PHP, and to throw an exception on intersection type.