Support for parallelism

Add support for parallelism via 3 new global environment variables:

RSPEC_RERUN_PARALLEL_NODE_TOTAL sets the total number of nodes for tests to be distributed across
RSPEC_RERUN_PARALLEL_NODE_INDEX is the current node
RSPEC_RERUN_PARALLEL_RANDOMIZATION controls whether test files will be randomized prior to distribution. This is to evenly spread slow tests (which are often in the same directory) across all nodes.