Add rule tester to CLI

[clockworksoul]

A rule tester would be helpful for rule development. Maybe something like:

$ gort evaluate --user admin \
  --rule 'with arg[0] == "user" must have gort:manage_users' 
  --command 'gort user list'

Could respond with something like:

{
  "result": true
}

or, alternatively:

{
  "result": false,
  "error": "failed to parse rule"
}