CLI: add LLM support

[casperdcl]

• please merge CLI: tidy #84 first
• add --llm-model
• add --llm-client-url
• add --llm-client-header

Tested successfully using https://ollama.com/library/llava-phi3:

OPENAI_API_KEY=ollama \
markitdown tests/test_files/test.jpg --llm-client-url=http://localhost:11434/v1 --llm-model=llava-phi3

Note that:

• --llm-client-header is needed for e.g. authori[sz]ation headers
• I didn't add --llm-client=OpenAI becuase there are no other options yet
• I hope MS will merge this despite it being a free alternative to its paid LLMs ;)