Expose verify_signature to allow verifying outside of the update call

[kalvenschraut]

https://github.com/jaemk/self_update/blob/master/src/update.rs#L331 I think this function just needs to be marked as public.

The example provided in the readme allows for doing all the parts of the update process independently of the update function outside of verify'ing the signature

I have two separate binaries, an installer and service. I need to do the manual steps in the installer when it first downloads the service to install on the machine, but only way to verify the signature is to reimplement that functions logic unless I am missing something.