Add a shorthand for AppendInvoke

[burdiyan]

I really like the idea of catching returned errors from deferred functions. Though having to use multierr package name twice in the same line makes it a bit verbose in many occasions.

This PR introduces a shorthand for AppendInvoke which allows passing function or method value directly without wrapping it into an Invoker.

So this:

defer multierr.AppendInvoke(&err, multierr.Invoke(my.StopFunc))

could become this:

defer multierr.AppendFunc(&err, my.StopFunc)

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #65 (d260cb7) into master (80b07a7) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #65   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          108       109    +1     
=========================================
+ Hits           108       109    +1     

Impacted Files	Coverage Δ
error.go	100.00% <100.00%> (ø)

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[sywhang]

Thank you for the contribution! This seems like a valid API to add to multierr. I left a couple of suggestions re: the doc. Can you please also add some test cases that covers this?

[burdiyan]

Applied suggestion docs changes. Added a test for the function. Let me know if you see other things to improve.

[sywhang]

Thanks for making the changes. LGTM.

[sywhang]

@abhinav do you know why the build is stuck here?