ENH: Create decorator to handle instantiating Shodan object

[rmhowe425]

Proposing the idea to convert get_api_key() in shodan.cli.helpers.py to a decorator that retrieves a user's API key and instantiates an instance of shodan.Shodan()

Proposed Implementation:

• Proposed changes to existing functionality for get_api_key() shown below.
• Add get_shodan_inst() decorator to all Shodan CLI commands.
• Add positional api parameter to all Shodan CLI commands.
• Remove function call to get_api_key() from Shodan CLI command function bodies.

Benefits:

• Significantly reduces redundant code.
• Improves overall readability of the code base.