#63 add context rule

[rchowinfoblox]

ABAC-policy requires this context rule stanza:

context {
    where ctx.tenant == "acme.com";
} {
    allow subject user pete.rose@chicago.il.us to buy products.inventory where ctx.genre == "gambling";
}

ie: we want to take the customer-written SEAL policy and surround it with account_id context as ABAC policies are specific to an account:

context {
    where ctx.subject.account_id == "42";
} {
    allow ...
}