feat: added NewLettersFromStringWithRGB

[floaust]

Description

Added NewLettersFromStringWithRGB.

Scope

What is affected by this pull request?

• Bug Fix
• New Feature
• Documentation
• Other

Related Issue

Fixes #321

To-Do Checklist

• I tested my changes
• I have commented every method that I created/changed
• I updated the examples to fit with my changes
• I have added tests for my newly created methods

[codecov]

Codecov Report

Merging #322 (b5652f5) into master (8a3c436) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #322   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         1459      1473   +14     
=========================================
+ Hits          1459      1473   +14     

Impacted Files	Coverage Δ
bigtext_printer.go	100.00% <100.00%> (ø)

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