Add input validation to shodan convert command

[rmhowe425]

• Closes out #204
• Created validation.py where I stored 2 functions used as callback methods for the input file type and output format for the shodan convert command.
• Functions used for input validation are passed in via the callback parameter for click.argument.

[rmhowe425]

@achillean If you're okay with this PR and my method for adding input validation, I would he happy to open a new issue & PR to add similar input validation for all other CLI arguments.

This could help clean up and simplify the actual implementation for each command.

[achillean]

The validation of the filename looks good but I'm not sure what the validation of the output parameters adds. click already checks that the output format is one of the values specified in type:

$ shodan convert test.txt nonsense
Usage: shodan convert [OPTIONS] <input file> <output format>
Try 'shodan convert -h' for help.

Error: Invalid value for '<output format>': 'nonsense' is not one of 'kml', 'csv', 'geo.json', 'images', 'xlsx'.

Also I'm not a fan of having to maintain the list of supported output formats in different locations. It's fairly rare that they get changed but it would be nicer if everything (help text, error text, mapping of output format to class etc.) was automatically generated. However, the primary hesitation for me in adding the callback for the output format is that I'm not sure why it needs to exist.

[rmhowe425]

Ah yep I see what you mean! I'll remove the callback input validation for the output format 😄

[rmhowe425]

@achillean I updated the implementation to only include filename input validation.