feat: Add support for global fallback function

[rarruda]

for reference:

• Unleash/unleash-python-sdk@af0bcd6
• Unleash/unleash-java-sdk@1f18286

[rarruda]

Actually this was reported in issue #33 and implemented in PR #40

[rarruda]

Sorry, it was implemented the isEnabled fallback function, not a global one.

Note that the references above seem to be wrong.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.