feat: add support for gpt-4o tokenizer encoding

[yaohui-wyh]

You can specify the encoding with the --encoding flag:

$ ctoc --encoding=o200k_base .
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Language                     files          blank        comment           code           tokens
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Go                              16            286            155           2113          22109
Markdown                         1             42              0            183           2596
YAML                             1              0              0             40            237
Makefile                         1              7              0             19            150
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TOTAL                           19            335            155           2355          25092
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