Add some help

[flawrist]

Imagine I downloaded this app once, then forgot about it, then rediscovered on disk.
Obvious questions are: what it does, is it safe (any dry-run switch), how fresh it is, where to get updates…

$ xo.exe & xo.exe --version & xo.exe --help

Usage: xo '/<pattern>/<formatter>/[flags]'

panic: runtime error: invalid memory address or nil pointer dereference    
[signal 0xc0000005 code=0x0 addr=0x30 pc=0x4010f5]                         
                                                                           
goroutine 1 [running]:                                                     
main.main()                                                                
        /Users/EzekielGabrielse/Go/src/github.com/ezekg/xo/main.go:24 +0xb5

panic: runtime error: invalid memory address or nil pointer dereference    
[signal 0xc0000005 code=0x0 addr=0x30 pc=0x4010f5]                         
                                                                           
goroutine 1 [running]:                                                     
main.main()                                                                
        /Users/EzekielGabrielse/Go/src/github.com/ezekg/xo/main.go:24 +0xb5