Add useragent feature

[lotusotho]

Adds the feature of changing your User-Agent to whatever you want.

I added the command -u or --useragent inside goop.go and a new method called CustomUE inside clone.go to check if the variable useragent is empty or not, basically encapsulating c variable body inside a function.

I think it would be neat to have this bc being able to change your user agent while enumerating a remote server gives you a lot more power by obfuscating your identity.