Add username generator feature

[kx-chen]

Add functionality that allow users to generate usernames.

Closes https://github.com/manu-chroma/username-availability-checker/issues/111

Uses the following library: https://github.com/awesmubarak/username_generator_cli

Quick screenshot of it:

Definitely need suggestions on how to improve this!

[seeeturtle]

Why don't you put them in the home page, instead of the result page. I think users who need that functionallity are unlikely to go inside the result page.

[kx-chen]

The username generator has been moved to the home page. Please suggest more changes :)

[manu-chroma]

Thank you for your contribution @kx-chen