Adds the ability to group options together, similar to command
groups. The parser groups semantics are:

1. Consecutive duplicate groups are merged.
2. Non-consecutive duplicate groups are __not__ merged.
3. Nested groups are concatenated together e.g. if "Group B" is parsed
   within "Group A", then B's options will render under
   "Group A.Group B".

1 and 2 is the same behavior as Command groups.

The (<<+>>) operator was swapped for (<</>>) in c1bf93b to fix pcapriotti#360. This was
mistakenly reverted when implementing the new rendering logic for option
groups. This commit re-establishes the fix.

Fixup help text.
Move worker function to where it's used.
Remove Applicative do from test suite.
Small tidies.

It is possible that someone defines a "parent group" that only contains
child group(s) i.e. it does not contain any top-level options. This is
a problem, because the parent name will not be attached to any
options, thus will not be printed.

To fix this, we instead represent the Option Groups as a hierarchy
list of group titles e.g. ["Parent 1", "Parent 2", "Child group"].

Then, when rendering, we keep track of which group titles have already
been printed. If some group has parent titles that have not yet been
printed, we prepend their titles to the normal output.