Optional snapshot root directory gradle property

[brenpearson]

Allows the consumer of the Paparazzi library to specify where they want the snapshots to be saved. This includes creating a gradle plugin extension for Paparazzi with this property in it, we can add other configurable properties to this extension in future.

This can be set as shown in the consumer's gradle file

paparazzi {
    snapshotRootDirectory = file("where/I/want/the/snapshots")
}