Display `all.equal` output

[brodieG]

Right now, we only do so for summary:

> summary(diffPrint(1:3, c(1,2,3) + 1/1e7))

No visible differences, but objects are not `all.equal`:
- Mean relative difference: 5e-08

> diffPrint(1:3, c(1,2,3) + 1/1e7)
No visible differences between objects, but objects are _not_ `all.equal`.
< 1:3        > c(1, 2,..
@@ 1 @@      @@ 1 @@    
  [1] 1 2 3    [1] 1 2 3