Named snapshots

[dylanbr]

As per the issue I previously already, here is an implementation of named snapshots, option 2.

Having the option to use explicitly named IDs for snapshots allows one to avoid needing read confusing diffs when a test is refactored (see the issue for an example).

This PR implements the following changes:

• Moves the incrementor functionality to the SnapshotIdAware concern. It makes sense to have it here, rather than in MatchesSnapshots, so getting an ID and incrementing are all a single operation.
• Added an optional $id argument to all the assertions. The retains backwards compatibility, while allowing explicit IDs to be used when required.
• When the optional $id is populated, this is used to name the snapshot, instead of the next incrementor value. It is also prefixed with s- to avoid the possibility of conflicts with incrementor values.
• Adds additional tests for all the assertions with the optional $id argument set.