Added default configuration

[mshrwtnb]

This PR enables ALRT to use defaultConfiguration.
By setting defaultConfiguration, ALRT can ...

1. Show an UIAlertController with user-defined tint color
2. Show OK and Cancel buttons with user-defined titles

See https://github.com/mshrwtnb/ALRT/pull/15/files#diff-f748f6c5192632893abcb48efd218a3fR105-R140 to understand how to use defaultConfiguration and its expected behavior.

[codecov]

Codecov Report

Merging #15 into master will decrease coverage by 0.16%.
The diff coverage is 75.60%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
- Coverage   71.59%   71.42%   -0.17%     
==========================================
  Files           1        1              
  Lines          88       77      -11     
==========================================
- Hits           63       55       -8     
+ Misses         25       22       -3     

Impacted Files	Coverage Δ
ALRT/ALRT.swift	71.42% <75.60%> (-0.17%)	⬇️

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