control: convert single tier to metered+per_unit

[bmizerany]

This changes how Tier interprets and converts tiers to Stripe prices.

Previously, a single tier in a pricing.json would convert to a metered
price with one or two tiers depending on if the tier had an inf limit or
not. If it did not have an inf limit, a catch-all was created to satisfy
Stripe. This produced awkward invoices.

This commit changes Tier to convert a feature that has a single tier
without a base price to be a metered+per_unit price in Stripe, which
shows up less awkwardly on invoices.

This also opens us up to support pricing transforms in Stripe for these
types of prices.

[bmizerany]

To test:

; cat <<-EOS | tier push -c -
{
    "plans": {
        "plan:tiers@0": {
            "features": {
                "feature:single": {
                    "tiers": [
                        {
                            "upto": 1,
                            "price": 1.1
                        }
                    ]
                }
            }
        }
    }
}
EOS

; tier subscribe org:single plan:tiers@0
; tier report org:single feature:single 10

You should see: