Deletion callback; replace edited mention; start mention without " " before

[tertmax]

• Added a callback that helps with handling of the deleted mention.
• Added a flag 'replaceEditedMention'. With it set to true, the edited mention will be fully replaced with a new one. The default value is false in order to preserve original behaviour.
• Added a flag 'considerTextBefore'. With it set to false, mentioning can be triggered without " " before the trigger symbols. The default value is true in order to preserve original behaviour.

[codecov]

Codecov Report

Merging #136 (59231a4) into master (967d7d9) will decrease coverage by 0.34%.
The diff coverage is 87.50%.

@@             Coverage Diff             @@
##            master     #136      +/-   ##
===========================================
- Coverage   100.00%   99.65%   -0.35%     
===========================================
  Files           21       21              
  Lines         1003      876     -127     
===========================================
- Hits          1003      873     -130     
- Misses           0        3       +3     

Impacted Files	Coverage Δ
Classes/Internal/String.swift	95.00% <80.00%> (-5.00%)	⬇️
Classes/MentionListener.swift	98.83% <89.47%> (-1.17%)	⬇️
Classes/Internal/Operators.swift	100.00% <0.00%> (ø)
Classes/Internal/MentionsArray.swift	100.00% <0.00%> (ø)
Classes/Internal/NSAttributedString.swift	100.00% <0.00%> (ø)
SZMentionsSwiftTests/DelegatesTests.swift	100.00% <0.00%> (ø)
SZMentionsSwiftTests/CreateMentionTests.swift	100.00% <0.00%> (ø)
SZMentionsSwiftTests/AddingMentionsTests.swift	100.00% <0.00%> (ø)
... and 4 more

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