make auth option to optionally accept auth token as input string

[ehsandeep]

Follow up for #307

As of now users can authenticate using interactive -auth option or ENV variable, optionally can we parse if user directly passing the api key as string input!

For example:

nuclei -auth # will prompt interactive option. 
cvemap -auth XXXXX # will parse string input as API key.

This is useful for automation where ENV support is not an option.