Allow configuring custom slack

[rabbbit]

Resolves #20

This is to allow people to configure custom slack - we previously
defaulted to 10, and only allowed to disable it.

The only tricky part about this is handling "per" correctly - adding
a bunch of test cases to cover it.

The tests are large, but when I tried to generate them dynamically
(the same test cases 2*, for both with-per and without-per) it became
even harder to read.

[codecov]

Codecov Report

Merging #64 (8b1d825) into main (7e71567) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #64   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           69        70    +1     
=========================================
+ Hits            69        70    +1     

Impacted Files	Coverage Δ
ratelimit.go	100.00% <100.00%> (ø)

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[rabbbit]

I also had the same test generated dynamically:

	const (
		slowRate = 10
		fastRate = 100
	)

	// Test for a few Per settings.
	testsPer := []time.Duration{
		// Default.
		{time.Second},
		{time.Millisecond * 500},
	}
	// And a few different slack options
	testsSlack := []int{10, 20, 100, 150}

	for _, per := range testsPer {
		for _, slack := range testsSlack {
			testKey := fmt.String("per-%v-slack-%v", per, slack)
			t.Run(testKey, func(t *testing.T) {
				runTest(t, func(r testRunner) {
					slow := r.createLimiter(slowRate, WithoutSlack)
					fast := r.createLimiter(fastRate, tt.opt...)

					r.startTaking(slow, fast)

					r.afterFunc(2, func() {
						r.startTaking(fast)
						r.startTaking(fast)
					})

					// slow limiter with 10hz dominates here - we're always at it's limit.
					r.assertCountAt(1*time.Second, slowRate)

					// results:
					// - for two seconds we ran a slow rate
					// - for a second we ran a fast rate
					// - and we add the slack
					// rates have to be adjusted for Per
					adjustedSlow := slowRate * int(time.Second/testsPer)
					asjustedFast := fastRate * int(time.Second/testsPer)
					r.assertCountAt(3*time.Second, asjustedSlow+adjustedFast+slack)
				})
			})
		}
	}

but it seems like manual definition is easier to understand at this point.

[cinchurge]

LGTM!