Add ability to specify certificate to use when signing the installer

[volks73]

Currently the /a flag is used with the signtool application for signing installers. This is fine most of the time as it indicates the application will automatically search for a suitable certificate in the Windows certificate manager. However, there are edge cases where someone may have multiple "suitable" certificates and want to specify a specific one to use. So, a -c,--certificate option should be added that takes a path and/or name to certificate to use when signing an installer with the -s,--sign flag.

[volks73]

I am not sure how this is supposed to work with passwords. I need to think about this a little bit.