allow custom user-agents

[1jz]

Add feature requested in #7

[tonyvugithub]

@1jz : hi Phillip, great contribution as always. Can you make a little adjustment to your PR for me?
In linkDetector.go
can you change line 61-64 from this

var userAgent string
checkCmd.StringVar(&userAgent, "u", "Go-http-client/1.1", "custom user-agent")

to this:
userAgent := checkCmd.String("u", ""Go-http-client/1.1", "custom user-agent")

I want to keep it consistent with the other flags. I think String() would return a pointer to the string flag which is basically what you have tried to pass in in StringVar(). Thank you. Update it and I merge it in.