Add --reverse option

[jgberry]

A proof of concept for a --reverse option as mentioned in #18. There is still some more work to be done here, but I wanted to open a draft to foster discussion on the approach I'm using here. Surprisingly, it seems this is a fairly straightforward addition.

[jgberry]

One caveat with this feature is that it may create unexpected orderings of global variables. For instance, it is common practice to declare module constants and global variables at the top of a file. However, when using the --reverse option these declarations tend to drift downwards so that they are declared after their usages.

Imports can encounter a similar issue as mentioned in #18.

[bwhmather]

I should warn you ahead of the time that the bar for this being merged is quite high. ssort was started as a reaction to others trying and repeatedly failing to manually sort modules in reverse order in a consistent way.

There is also the question of whether we want to risk splitting the ecosystem. There is something to be said for the any-colour-you-like approach.

That said, copy the samples tests and let's see what the output looks like on real code.

[jgberry]

Ok, given your insights and some quirks I discovered, which I captured in failing unit tests in the above commits, I think it would be best to put this work on hold for now.

[jgberry]

Closing this draft for now.