Subtest note at start

Talking with @Arc just now, he repeated an oft requested feature that a subtest would be more readable if it had its name at the beginning.  I'm resurrecting #290 written by @SineSwiper with the small change that the subtest note is indented with the block.
```
  1..3
  ok 1 - First test
      # Subtest: An example subtest
      1..2
      ok 1 - This is a subtest
      ok 2 - So is this
  ok 2 - An example subtest
  ok 3 - Third test
```

PS  Contrary to what I thought in #290 a year ago, there is going to be another stable release before 1.5.

@SineSwiper @Arc What do you think?