Made a new function sort_by(lines, compare_fns), which accepts a
list of compare_fns and calls lines.sort_by() with a closure that
calls each compare_fn in turn until one returns something other
than equal.
Default behavior ensures that String::cmp is the last element in the
compare_fns list (referred to as 'last resort' sorting by man sort).
Passing --stable (-s) turns this behaviour off.
Test cases provided for `sort --month` and `sort --month --stable`.
Instead of using numerals to denote individual cases, I used descriptive
case names. I also changed the extension for the expected output fixture
to match other tests.
I removed one redundant test and another unnecessary helper function.
The repetition of "foo" and "bar" made for difficult-to-read assertion failures
when hacking on `tail`. I think that having each line have unique contents makes
it a bit easier to parse.
