feat: add prompt functionality to Whisper engine

[ken0911208818]

• Add a new field Prompt to the Whisper struct in config/whisper.go
• Add a new flag prompt to the command line interface in main.go
• Set the Prompt field of the Engine struct in whisper/whisper.go

[codecov-commenter]

Codecov Report

Merging #10 (f1bb8bb) into main (0da479b) will decrease coverage by 0.03%. The diff coverage is 0.00%.

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@@           Coverage Diff            @@
##            main     #10      +/-   ##
========================================
- Coverage   7.53%   7.50%   -0.03%     
========================================
  Files          5       5              
  Lines        252     253       +1     
========================================
  Hits          19      19              
- Misses       231     232       +1     
  Partials       2       2              

Files Changed	Coverage Δ
whisper/whisper.go	8.60% <0.00%> (-0.06%)	:arrow_down: