Unify login management under one subcommmand

[rossengeorgiev]

Currently every subcommand prompts for login information when needed.
With the growing number of subcommand that is annoying, especially when trying to use steamctl as part of script

• New subcommand steamctl login, which will handle all login prompts.
• Subcommands that require an account will either success or fail, no prompts
• A failure will require interaction with steamctl login to get a working set of credentials
• steamctl login will support environmental variables for ease of use in scripts

[ChanceHarrison]

@rossengeorgiev Are you open to contributions pertaining to this issue?

[rossengeorgiev]

hi @ChanceHarrison, yes, all PR welcome. The requirements are listed here, and the command need to fit the current style commands.

[nutterthanos]

#56