Clean up the initial generator Mix task

[waseem-medhat]

Summary of changes:

• Added documentation
• Two identical functions MixHelpers.fetch_repo_contents and MixHelpers.fetch_dir_contents were replaced with a single one MixHelpers.fetch_gh_contents!, with the added ! convention
• Renamed fetch_content_object to use the word "download" which makes the distinction between fetching functions (returning the GH API response as-is) vs. this function saving the files to the file system
• Reformatted some of the output messages
• Removed the TerminalHelpers module, since it's not very beneficial, and simple direct calls to std lib functions in IO.ANSI or Mix.Shell.IO should be good enough

This may need additional refactoring once we start adding exercise fetching from local paths. But for now this should close #5

[waseem-medhat]

I usually leave them for clarity, but on a second look I agree. They don't communicate that much in this case. So, I removed them.

Glad you like the rest of it though. Thanks!