Support Perl module hooks

Currently, mylvmbackup supports calling "hooks" at various stages of the process. At present, these can be external executables of any kind (e.g. shell scripts, binaries, etc.). In addition to that it would be nice to be able to load external hooks written as Perl modules that would then inherit global variables, the database connection handle and configuration options. For this the run_hook() function needs to be updated to look for <hookname>.pm and load the respective module, if it exists.