Add ANSI support to console logger

* Coloring is off by default because I could not think of a way to choose a default with `IO.ANSI.terminal?/1`
* Advanced coloring can be done by the formatter for `:console` backend because the formatter can return a value that is passed to `IO.ANSI.format/2` (chardata with ANSI atoms)
* `:console` backend does smart options merging so the old `:colors` options are not lost
Maybe we should not pass the data to `IO.ANSI.format/2` to avoid traversing all data? I can fix this tomorrow unless you have an objection.