fix --no-color / --color option for `mix test`

The `--no-color` flag builds up an options hash to include `[color: false]` which is then passed to `ExUnit.configure/1`. The ExUnit application isn't aware of a `:color` attribute, but it does know of a `:colors` attribute which takes a list of colors to be used by formatters.

This PR will convert the `:color` flag given to the `mix test` task into the proper option format that `ExUnit.configure/1` expects.
We cannot default it to true because it doesn't work on systems like Windows. We need to check if --color was explicitly set and add its value only if so.