[Enhancement] Update `Mix.Tasks.Bio.Random.Dna`

[pcapel]

There are a few pain points when using this task:

1. Documentation is poor. It can be updated to use better formatting etc.
2. The error give when no file is present is derived from the File.write function and not clear enough. The module should check for a missing argument if it is required.
3. It seems like the way it's checking for arguments is a little bit off. Maybe update that.