Structure for re-usability and concurrency

[System-Glitch]

Description

Create a structure and make it implement the slug functions.

Benefits:

• Ability to slug concurrently with different settings (this wasn't possible because of global variables)
• Better re-usability

Drawback:

• Very slightly slower due to map cloning when creating a new Slug structure.

Example usage

s := slug.New()
s.MaxLength = 80
s.Make("Hello world")

Notes

This change is not breaking.

Similar to #41

[codecov]

Codecov Report

Merging #51 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #51   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           84        83    -1     
=========================================
- Hits            84        83    -1     

Impacted Files	Coverage Δ
slug.go	100.00% <100.00%> (ø)
languages_substitution.go	100.00% <0.00%> (ø)

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