Cleanup; Add ZIP and codesign options

Hello!

Thanks for the useful utility!

I've cleaned the code a bit to follow idiomatic Go style, and added these options:

 -z -- if true, will output ZIP files (in binary_os_arch.zip format) instead of directories with binaries inside

 -codesign="identity" -- runs "codesign" command with the specified identity on OS X to sign binaries