Reversed stack ordering

[jonhoo]

We should support the --reverse option, which, well, reverses the order of all stacks. Note that this does require that we also sort our input, because otherwise the trick we play in flow in flamegraph/merge.rs doesn't work!

[jasonrhansen]

#96 made a change where we sort input lines by default. I was looking into adding the --reverse option and it made me aware of something a little weird in our current API. We've added the no_sort option where you can say your input is already sorted. The problem is, it doesn't make any sense to use this option with from_sorted_lines since it already expects the lines to be sorted. That's not so bad on its own, but if we add the reverse option, and our sorting happens in from_reader and from_readers, before from_sorted_lines lines is called, then setting the reverse flag and calling from_sorted_lines directly will not reverse the stacks.

I propose we change from_sorted_lines to simply from_lines and do the sorting and stack reversing there. The caller can still set the no_sort flag if the input is already sorted. They can also use the reverse flag with from_lines. This way these options make sense no matter which function you call into.