Compare command

This is just a very experimental idea for a 'compare' command that can be used to compare two specific revisions, e.g:

```
$ asv compare 7810d6d7 19aa5743
· Fetching recent changes.

All benchmarks:

  before     after    ratio
  98.30ms   68.19ms      0.69  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_ann_center
 139.58ms  108.97ms      0.78  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_ann_exact
  98.94ms   67.96ms      0.69  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_ann_subpixel_01
 105.35ms   73.63ms      0.70  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_ann_subpixel_05
 116.30ms   86.23ms      0.74  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_ann_subpixel_10
  48.15ms   51.00ms      1.06  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_center
  68.18ms   69.50ms      1.02  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_exact
  48.59ms   51.24ms      1.05  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_subpixel_01
  51.13ms   53.71ms      1.05  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_subpixel_05
  56.26ms   58.65ms      1.04  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_subpixel_10
    4.71s  231.49ms      0.05  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_elli_ann_center
    1.70s     1.67s      0.98  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_elli_ann_exact
    4.72s  242.42ms      0.05  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_elli_ann_subpixel_01
   failed  454.41ms       n/a  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_elli_ann_subpixel_05
   failed     1.13s       n/a  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_elli_ann_subpixel_10
...
```

Benchmarks are color-coded as green if they are faster by a given threshold, and red if they are slower. The threshold can be adjusted with the ``--threshold`` option.

The output can be split between benchmarks that got better, didn't change, or got worse, with the ``--split`` option:

```
$ asv compare 7810d6d7 19aa5743 --split -t 1.5
· Fetching recent changes..

Benchmarks that have improved:

  before     after    ratio
    4.71s  231.49ms      0.05  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_elli_ann_center
    4.72s  242.42ms      0.05  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_elli_ann_subpixel_01
   failed  454.41ms       n/a
[snip]
   failed  159.65ms       n/a  benchmarks.AperturePhotometry.time_small_data_multiple_small_apertures_elli_subpixel_05
   failed  215.41ms       n/a  benchmarks.AperturePhotometry.time_small_data_multiple_small_apertures_elli_subpixel_10

Benchmarks that have stayed the same:

  before     after    ratio
  98.30ms   68.19ms      0.69  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_ann_center
 139.58ms  108.97ms      0.78  benchmarks.AperturePhotometry.time_big_data_error_multiple_big_apertures_circ_ann_exact
  98.94ms   67.96ms      0.69
[snip]
   1.55ms    1.52ms      0.98  benchmarks.AperturePhotometry.time_small_data_single_small_aperture_elli_subpixel_05
   1.72ms    1.58ms      0.92  benchmarks.AperturePhotometry.time_small_data_single_small_aperture_elli_subpixel_10

Benchmarks that have got worse:

  before     after    ratio
   2.30ms    3.71ms      1.61  benchmarks.AperturePhotometry.time_big_data_single_big_aperture_elli_center
   2.31ms    3.71ms      1.61  benchmarks.AperturePhotometry.time_big_data_single_big_aperture_elli_subpixel_01
```

I implemented this because I needed it for a specific set of benchmarks where I'm trying to compare the impact of different optimization strategies, but of course you can reject this or make suggestions for significant changes, as needed.
Maybe I'm just not understanding -- with the default threshold of "2", should we see all of these results?  I would think the 0.66, 0.52, 1.0, 0.93, 0.70 and 1.10 ratios would all be hidden...