Add clear history cli cmd

[halfnibble]

After performing content safety testing on various models, I realized it would be nice to clear the history.

Not sure if this warrants an api handler func like the other handlers?

Also, I thought about abstracting the history path code.

[halfnibble]

Here is the serve version. I think this is close. https://github.com/halfnibble/ollama/tree/add-clear-serve

Let me know if this implementation is closer to ideal, and I'll update the PR. (I'm not too familiar with this pattern of cli calling a local api for everything)