Expose custom strategy methods in Gradle plugin

[ajoberstar]

50 introduced more powerful ways to configure the strategies, having the vcs inventory available as part of determining the scope or stage. This flexibility should be available within the Gradle plugin.

Add the following methods to ReckonExtension:

• scopeFrom(Function<VcsInventory, Optional<String>>)
• stageFrom(BiFunction<VcsInventory, Version, Optional<String>>)
• snapshotFrom(BiFunction<VcsInventory, Version, Boolean>)

[ajoberstar]

See #70 for a use case of this.

[ajoberstar]

Want to think about some of the use cases for this before supporting it.

[ajoberstar]

See #98 for another use case for this.

[ajoberstar]

Fixed by #172