Add functional options for NewCollector

[peterhellberg]

Issue #80

[codecov-io]

Codecov Report

Merging #82 into master will increase coverage by 3.78%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #82      +/-   ##
=========================================
+ Coverage   43.01%   46.8%   +3.78%     
=========================================
  Files           7       7              
  Lines         744     797      +53     
=========================================
+ Hits          320     373      +53     
  Misses        385     385              
  Partials       39      39

Impacted Files	Coverage Δ
cmd/colly/colly.go	46.93% <0%> (+6.22%)	⬆️

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[asciimoo]

👍
Wow, that was fast and the code is clean and understandable.
Thank you for your valuable contribution.