updating the db file handling to follow the Sanford pattern

Define a config for the db file, default it with an ENV var.  Then
require in the db file on `init`.

This updates Ardb to this pattern - the goal is to be as similar
to Sanford as possible.  The reason for this is they are both
gems, with CLIs, that need some configs in an expected file, and then
do stuff.
Merge pull request #36 from redding/kr-env

updating the db file handling to follow the Sanford pattern