implement sort --stable

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`.

tests/fixtures/sort/month_default.expected

@@ -3,5 +3,8 @@ Jan Lorem ipsum dolor sit amet
 mar laboris nisi ut aliquip ex ea
 May sed do eiusmod tempor incididunt
 JUN nostrud exercitation ullamco
+Jul 1 should remain 2,1,3
+Jul 2 these three lines
+Jul 3 if --stable is provided
 Oct ut labore et dolore magna aliqua
 Dec consectetur adipiscing elit

tests/fixtures/sort/month_default.txt

@@ -5,3 +5,6 @@ Oct ut labore et dolore magna aliqua
 N/A Ut enim ad minim veniam, quis
 JUN nostrud exercitation ullamco
 mar laboris nisi ut aliquip ex ea
+Jul 2 these three lines
+Jul 1 should remain 2,1,3
+Jul 3 if --stable is provided

tests/fixtures/sort/month_stable.expected

@@ -0,0 +1,10 @@
+N/A Ut enim ad minim veniam, quis
+Jan Lorem ipsum dolor sit amet
+mar laboris nisi ut aliquip ex ea
+May sed do eiusmod tempor incididunt
+JUN nostrud exercitation ullamco
+Jul 2 these three lines
+Jul 1 should remain 2,1,3
+Jul 3 if --stable is provided
+Oct ut labore et dolore magna aliqua
+Dec consectetur adipiscing elit

tests/fixtures/sort/month_stable.txt

@@ -0,0 +1,10 @@
+Jan Lorem ipsum dolor sit amet
+Dec consectetur adipiscing elit
+May sed do eiusmod tempor incididunt
+Oct ut labore et dolore magna aliqua
+N/A Ut enim ad minim veniam, quis
+JUN nostrud exercitation ullamco
+mar laboris nisi ut aliquip ex ea
+Jul 2 these three lines
+Jul 1 should remain 2,1,3
+Jul 3 if --stable is provided