add new sampling algorithm "mirostat"

[h3ndrik]

This makes the (new) mirostat sampler from ggml-org#1126 available. There are more sampling techniques and features in that pull request.

I'm not sure if i implemented this correctly, so have a look at it before merging ;-) This only applies to llama.

[LostRuins]

this implementation is incorrect. Mirostat is a replacement for stochastic samplers and cannot be used alongside with them.

I plan to eventually add it as a command line flag, but I cannot support it within the Kobold API until it is implemented officially at https://github.com/henk717/KoboldAI

[LostRuins]

I ended up merging it, but converting it to a launcher flag instead.