Allow parameterising secret_key and ttl

[bjeanes]

This addresses #1 but with a different approach, which minimises the diff while maintaining backwards compatibility.

This allows context-specific keys to be used for encoding and decoding as well as context-specific ttls:

key = SecureRandom.bytes(32)
ciphertext = Branca.encode("sensitive information", secret_key: key)
Branca.decode(ciphertext, secret_key: key, ttl: 30)

[bjeanes]

I also made it so that DecodeError was raised. Though I realise this probably would NOT be a backwards compatible change and so could be removed. However, I saw that you had the exception defined but were not using it so assumed this to be your original intent.

[thadeu]

@bjeanes Amazing your changes, simple and powerful! Keeping backwards compatible with other versions.

I added build and tests on gh actions in this repository in order to help our changes

[thadeu]

LGTM