Drop callback support

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Removes callback-style support (use nodeify if you need that still), and adds an async/await example to the API docs.

[codecov]

Codecov Report

Merging #916 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #916   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         671    668    -3     
=====================================
- Hits          671    668    -3

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #916 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #916   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         671    668    -3     
=====================================
- Hits          671    668    -3

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 41bf218...6b7579b. Read the comment docs.