Add configuration options to override Benchee default values

Benchee is getting a lot of configuration options - which is great. But the more configurations options and formatters there are, the more people would probably prefer to configure some of them globally for their project.

Something like:

`config :benchee, :options, %{} # fancy map or keyword list overriding default options`

These options should then represent the new default options. So merge order would be something like: default_config <- app_config <- benchmark_config (<- meaning right overrides left)