Add run once convention initialization block

[BernieWhite]

Currently convention blocks run n times. The specific number of times depends on how many objects or rules are processed.

For scenarios such as once off initialization there is no obvious or effortless way to run code once.

Add support for an additional block that runs before rules and objects are loaded and passed through the pipeline for initialization.

For example:

Export-PSRuleConvention 'Init' -Initialize {
    # Run once code here.
}