If parsing a table with keys/values, pass key to the converter

[mm-gmbd]

Would be handy if, when parsing a table in parse method, that the key be passed to the optional converter function.

[mm-gmbd]

@ppetrosh -- not sure if you're managing this library, but figured I'd give you a mention to get things started.

[mm-gmbd]

This ended up being a simple change -- just pass the available key variable as the 3rd argument to _convert method. Unfortunately this is a breaking change (requires others using the library to update so that converter functions take 3 args instead of 2).

[ppetrosh]

Hi Max do you have a PR for the change? Or can you share it with me so that I can take it over from there?

[mm-gmbd]

Hey Pavel -- opened PR #7