Add single-slot captions option

[shaiananvari8]

Summary

• add a --single-slot-captions boolean option
• when enabled, replace any currently active caption before showing the next one
• document the new option in the README and man page

Closes #344.

Validation

• git diff --check
• Build not run locally: this Windows environment does not have Gource's native build toolchain available, and WSL has no installed distribution.

Bounty note

This implements the single-slot caption behavior requested in #344. If accepted, I can coordinate the bounty payment method privately.