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.
@mdboom - thanks for the PR - I do think it would be nice to have a mode where results can all be compared irrespective of the threshold, so I like the option of keeping the current output plus the characters you are suggesting rather than filtering out only the ones that change a lot. Maybe the ``--split`` mode could remove the ones that don't change though?