Add to configuration the option to disable `--function-context` 

`--function-context` adds a lot of boilerplate in the prompt for each line change reaching easily the limit of `5k` token per commit.

We should provide the option developers disable it so that we support messages on longer commits too.

P.S. As a workaround developers can switch to GPT3 that supports 16k tokens too (model: `gpt-3.5-turbo-16k`)

Original reporter: @nikostoulas 