Error Checking: Validate logtype arguments

[drewsortega]

When the user provides log type, for example 'rdp', 'dns', etc, there is currently no error checking. Nagini just looks for files starting with this log type and if its not a real log type then it will not fail, and just say it finds no longs.

Instead, there should be error checking before starting. Some requirements:

• When log_type provided, remove all capitalization. For example, RDP -> rdp, DNS -> dns.
• Check if valid log_type. Since Zeek deployments can vary with valid log types, the best thing to do would be to do a directory list and compile a list of found log types, then compare to the input.