Add support for %verifyscript, %pretrans and %posttrans scripts for RPM packages.

Hi.

We found ourselves having the need to add these scripts to our RPM packages:
http://wiki.networksecuritytoolkit.org/nstwiki/index.php/RPM_Quick_Reference#Secret_.25pretrans_and_.25posttrans_RPM_Scriptlets

Unfortunately FPM was not supporting them.

So here is a PR to add support for those.