Set CUSTOM User-Agent for PHISHLETs

A custom_ua field has been added to each phishlet in the configuration. Allows users to specify a custom User-Agent, which will be injected into the HTTP request before sending it to the server.

Some websites deliver different content depending on the User-Agent, and manipulating the User-Agent can help bypass certain security measures.

Usage: Simply update the custom_ua field like so: phishlets custom_ua <phishlet> "<ua>"

Log output: NOTE: When a "new visitor has arrived" it will print the original User-Agent but will send to the server the custom one.'