Print the bash command that caused the test to fail

Includes the `$BASH_COMMAND` and exit status that caused the test failure in the pretty failure output

```
$ cat example.bats 
@test "a passing test" {
  true
}

@test "a failing test" {
  false
}

@test "another failing test" {
  [ true == false ]
}

@test "a failing test with variables" {
  X=1
  [ $X -eq 2 ]
}

$ bin/bats example.bats 
 ✓ a passing test
 ✗ a failing test
   (in test file /Users/ryansouza/p/bats/example.bats, line 6)
     'false' exited with status 1
 ✗ another failing test
   (in test file /Users/ryansouza/p/bats/example.bats, line 10)
     '[ true == false ]' exited with status 1
 ✗ a failing test with variables
   (in test file /Users/ryansouza/p/bats/example.bats, line 15)
     '[ $X -eq 2 ]' exited with status 1

4 tests, 3 failures

$ bin/bats -t example.bats 
1..4
ok 1 a passing test
not ok 2 a failing test
# (in test file /Users/ryansouza/p/bats/example.bats, line 6)
#   'false' exited with status 1
not ok 3 another failing test
# (in test file /Users/ryansouza/p/bats/example.bats, line 10)
#   '[ true == false ]' exited with status 1
not ok 4 a failing test with variables
# (in test file /Users/ryansouza/p/bats/example.bats, line 15)
#   '[ $X -eq 2 ]' exited with status 1
```

edit: rebased & updated (11/10)