Update command line interface

[fccoelho]

Update the command-line interface in skeleton.py, to expose new functionalities in the library.

This issue may remain continually open with new features being added and marked as completed in the following task list:

• show dependencies ( Implement get_dependencies #8 )
• dependency graph ( Implement dependency graph viewer #7 )
• package popularity ( package popularity #9 )
• Installed packages ( Implement listing of installed packages #2 )
• Upgradable packages ( List upgradable packages #3 )
• Show package releases ( )

[fccoelho]

To make progress on this issue and check the boxes, new issues must be open to implement the command-line argument to invoke each analysis.

@AliferSales : you can add more check boxes as new features get proposed. Just edit the original message.