Refactor LLMs to have optional parameters

[ChristianSch]

Quoting @tmc from #43 :

Let's do variadic/functional Options arguments to pass these (and have some reasonable defaults).

like

type Option func(LLM)

func New(options ...Option) (*LLM, error) {
    llm := &LLM{...}
    for _, o := range options {
       o(llm)
   }
}

/e: I wanted to also document which LLMs need to be updated, but as this is changing so quickly, I don't think that's feasible atm