CLI: Add support for manipulating individual ACLs

[vhartikainen]

We'd like to manipulate individual ACLs via CLI, instead of creating the new ACL in Consul GUI and then backup & restore with appropriate changes.

e.g.
consulate acl list -> for listing the current ACLs
consulate acl get -> for showing the rules of a given ACL
consulate acl create -> for creating a new ACL if it doesn't exist
consulate acl set -> for setting rules of an existing ACL
consulate acl rm -> for removing an ACL

[aseppala]

+1