Add metadata object on Credit Card charge creation

[zakiarsyad]

Currently, Xendit API for creating a charge allows metadata object to be sent in the request payload -> https://developers.xendit.co/api-reference/#create-charge

This library doesn't support metadata object yet https://github.com/xendit/xendit-python#create-charge

[msmur]

Resolved with #49
Released with https://github.com/xendit/xendit-python/releases/tag/v0.1.7