feature: adding the option to provide appender instance in config

[kaxelson]

This PR is related to issue #832.
It adds the ability to supply an appender instance (instead of just a string name) when specifying a configuration.

[codecov]

Codecov Report

Merging #833 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #833   +/-   ##
=======================================
  Coverage   97.43%   97.43%           
=======================================
  Files          25       25           
  Lines         937      937           
=======================================
  Hits          913      913           
  Misses         24       24

Impacted Files	Coverage Δ
lib/appenders/index.js	95.91% <100%> (ø)	⬆️

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