Change the argument names of pretty.Compare to a and b

Fixes #10

The Formatter field is a map from reflect.Type to arbitrary function
values that are callable with values of the given type and which return
a string.  The values will be printed literally.

Fixes issues: #8, #11

Remove padding from Diffable output

When comparing maps where the padding differs between the two sets of keys, the
diff will include keys that were unchanged except for the padding difference.

For example, Arthur Dent appears as a diff due to different padding:
```
{
  Crew: {
-        Arthur Dent:       "Along for the Ride",
-        Zaphod Beeblebrox: "Galactic President",
+        Arthur Dent: "Along for the Ride",
        },
 }
```

This omits padding when `Diffable = true` so that the diff output is consistent
to prevent these spurious diffs.

Allow the shortening of diffs by skipping fields with zero values

CompareWithoutZeroFields returns a diff like Compare but omits
struct fields that have a zero value.