Ignore non-config flags

[MaKleSoft]

Currently if I use configure.ParseCommandLine(config, os.Args[1:]) and run the program with a flag not declared in my config struct I get the error unknown long flag '--my-flag'. This obviously causes problems in case I want to support other flags apart from the ones declared in my config struct or even just use two separate structs. Therefore, I think it would be better if undeclared flags would simply be ignored.

[llorllale]

use two separate structs

I agree. I'm forced to break encapsulation between two unrelated packages due to this.