[FEATURE] Use username if no output directory is given

[Dekamir]

• I am requesting a feature.
• I am running the latest version of BDfR
• I have read the Opening an issue - doesn't exist

Description

While downloading, most users will download content, especially profiles, to a directory with the same name as the username. So the command usually goes like this:
bdfr download --user Dekamir --submitted Dekamir

If a username is given, or it can be pulled by the link, and the user hasn't given an output directory argument, use the username by default.
bdfr download --user Dekamir --submitted should output to ./Dekamir by default.