Formatting improvements.

* Scale iter counts by 10^3s with suffix, for better readability.
* Reduce printed precision to 3 decimal places throughout.
* Fix stdddev percentage width to 4 characters, for alignment.

Here's the outputs before and after. I was always having trouble reading ips counts when they get into the hundreds of thousands or higher, and the stddev alignment thing was just bugging me. The ips scaling change also helps keep alignment consistent.

The precision changes warrant some justification. For the time elapsed, I figured that anything lower than ms is not useful for an "iterations per second" benchmark, and the only times shown are all around 5s range. For i/s, anything scaled 1/1000 is going to be in the neighborhood of benchmark noise. If something can do 50M iterations in 5 seconds, that's 10M per second, and 1000 iterations (the 1/10000 lost precision) takes 0.1ms. The error rate for that benchmark is +/- 1000 iterations, or 0.0002%. Acceptable to have good, readable results.

BEFORE:
```
Calculating -------------------------------------
uncontended MD5 'foo'
                         29296 i/100ms
uncontended MD5 'foo' * 1000
                          9558 i/100ms
4x contended MD5 'foo'
                           599 i/100ms
4x contended MD5 'foo' * 1000
                           519 i/100ms
10x contended MD5 'foo'
                           267 i/100ms
10x contended MD5 'foo' * 1000
                           229 i/100ms
100x contended MD5 'foo'
                            21 i/100ms
100x contended MD5 'foo' * 1000
                            19 i/100ms
-------------------------------------------------
uncontended MD5 'foo'
                       543446.5 (±5.2%) i/s -    2724528 in   5.028890s
uncontended MD5 'foo' * 1000
                       134059.2 (±2.9%) i/s -     678618 in   5.066376s
4x contended MD5 'foo'
                       123911.7 (±10.6%) i/s -     609782 in   4.994339s
4x contended MD5 'foo' * 1000
                        31893.7 (±6.9%) i/s -     158814 in   5.006711s
10x contended MD5 'foo'
                        47335.5 (±9.4%) i/s -     233625 in   4.994032s
10x contended MD5 'foo' * 1000
                        12726.5 (±5.5%) i/s -      63433 in   5.001239s
100x contended MD5 'foo'
                         2463.1 (±8.1%) i/s -      12222 in   4.999318s
100x contended MD5 'foo' * 1000
                          976.0 (±5.2%) i/s -       4883 in   5.018057s
```

AFTER:
```
Calculating -------------------------------------
uncontended MD5 'foo'
                        29.733k i/100ms
uncontended MD5 'foo' * 1000
                         9.572k i/100ms
4x contended MD5 'foo'
                       618.000  i/100ms
4x contended MD5 'foo' * 1000
                       531.000  i/100ms
10x contended MD5 'foo'
                       273.000  i/100ms
10x contended MD5 'foo' * 1000
                       234.000  i/100ms
100x contended MD5 'foo'
                        22.000  i/100ms
100x contended MD5 'foo' * 1000
                        19.000  i/100ms
-------------------------------------------------
uncontended MD5 'foo'
                        553.452k (± 3.5%) i/s -      2.765M in      5.003s
uncontended MD5 'foo' * 1000
                        134.654k (± 2.9%) i/s -    679.612k in      5.051s
4x contended MD5 'foo'
                        124.699k (±10.1%) i/s -    614.292k in      4.994s
4x contended MD5 'foo' * 1000
                         32.174k (± 6.9%) i/s -    160.362k in      5.012s
10x contended MD5 'foo'
                         47.166k (± 9.2%) i/s -    233.142k in      4.999s
10x contended MD5 'foo' * 1000
                         12.643k (± 5.6%) i/s -     63.180k in      5.015s
100x contended MD5 'foo'
                          2.513k (± 7.2%) i/s -     12.496k in      5.002s
100x contended MD5 'foo' * 1000
                        978.344  (± 5.1%) i/s -      4.883k in      5.006s
```
I modified it to use @format in that first patch, but then decided that the jobs shouldn't really aggregate a format; they should get it from an external config source so it can be changed without changing the jobs. They all go after it via Benchmark::IPS.options now