Add automatic values to salt

[tbekolay]

Currently, instead of an integer seed_offset, pytest-rng uses a string salt value that is concatenated to the nodeid before hashing. I did this so that it would be easy to use Nengo Bones (specifically nengo/nengo-bones#39) to put the version string in there.

However, another way we could make the salt value change automatically is to recognize some special strings that will be expanded to a concrete string when pytest is run.

Here are some options for discussion:

1. Pass the salt string to strftime with the current time.

   This would enable things like rng_salt = "%Y-%m", meaning that you would get a new unique seed every month.

2. Come up with a list of custom values that the user can use in their salt string and pass them into a .format call.

   This would enable anything we want, including rng_salt = "{year}-{month}", but would require us to come up with a list of things we might want.

Other options welcome.