Fix snakecase converter for multi-numeric strings

[jespBE]

Resolves #516

Change to the SnakeCase letter converter to give more consistent behavior for strings containing multiple numeric characters in sequence. For instance:

print(old_snakecase('Alice'), new_snakecase('Alice'))
alice alice

print(old_snakecase('Alice123'), new_snakecase('Alice123'))
alice_1_2_3 alice123

print(old_snakecase('AliceBob123'), new_snakecase('AliceBob123'))
alice_bob_1_2_3 alice_bob123

[jespBE]

Closing because uploaded on wrong Github account, oops!