logger: implement logger option

[lopezator]

Add a Logger interface, along with a LoggerFunc and a new WithLogger option.

This would enable:

• Customizing migrator output
• Integrating a 3rd party logging tool

By default, migrator will work as before, just outputting to stdout.

Closes #6

[codecov]

Codecov Report

Merging #31 into master will decrease coverage by 3.68%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
- Coverage   77.41%   73.73%   -3.69%     
==========================================
  Files           1        1              
  Lines          93       99       +6     
==========================================
+ Hits           72       73       +1     
- Misses         13       18       +5     
  Partials        8        8              

Impacted Files	Coverage Δ
migrator.go	73.73% <66.66%> (-3.69%)	⬇️

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[lopezator]

Would this be enough for you @adamdecaf?

You could use WithLogger option along with LoggerFunc function, and then do nothing inside to avoid printing.

migrator, err := New(Migrations(migrations...), WithLogger(LoggerFunc(func(string, ...interface{}) {
    // Doing nothing here would disable the output
})))

CC/ @dufcrule @glerchundi

[adamdecaf]

This would work great for me, thanks for the quick change!

[adamdecaf]

It also works to reformat logs into JSON, thanks!

[lopezator]

That's great! Did you used some 3rd party tool for json formatting?

[adamdecaf]

We use the go-kit/kit logger which has a JSON outputter.

[lopezator]

That's great. Thank you for testing it out!

[dufcrule]

Just some minor changes to the README texts. Apart from that LGTM!

[lopezator]

Thank you for the review @dufcrule !!