Add support for specifying custom logger

[kibertoad]

fixes #7

[simoneb]

LGTM, can you please fix the failing test? It's due to CI getting a new version of fastify containing this change.

[kibertoad]

@simoneb Sure, give me a sec.

[codecov-io]

Codecov Report

Merging #11 (8c3c5de) into master (e8febb4) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #11   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           26        28    +2     
=========================================
+ Hits            26        28    +2     

Impacted Files	Coverage Δ
plugin.js	100.00% <100.00%> (ø)

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