Support v3 promises.

[derekrprice]

Community guidelines:

• [x] I have read and understand the contributing guidelines and code of conduct

This PR contains:

• [ ] bugfix
• [x] feature
• [ ] refactor
• [ ] documentation
• [ ] other

Are tests included?

• [x] yes (bugfixes and features will not be merged without tests)
• [ ] no

Breaking Changes?

• [ ] yes (breaking changes will not be merged unless absolutely necessary)
• [x] no

List any relevant issue numbers: #21

Description

Adds support for returning promises for functions accepting optional callbacks.

[aThorp96]

Thank you for starting this PR. This is sorely needed.

[tscislo]

Any chance for this to get merged in?