Added options for Sqlite backend

[pbv]

Hello,
I've added some options to allow using the Snap.Auth.Sqlite backend; maybe you'd like review these and merge them into your repository.

[dzhus]

Hey, thanks for the patch, I'm going to ask you to put in a few lines in README.md too to document this (as it no longer supports only JSON)

[pbv]

Before I do that let me point out that I did a quick hack and changed the options semantics slightly: instead of -j file you need to use -j -f file (for Json) or -s -f file (for Sqlite). I couldn't figure a way to parsing an enumeration with fields using implicit arguments and didn't want to rewrite the option parsing using a lower level library.
This may break existing scripts; if you're OK with this I'll modify the README.

[dzhus]

@pbv Of course, the change makes sense, and I think it's perfectly fine as long as it's documented. I'll use this as a chance to start a CHANGELOG too.

[pbv]

OK, I've updated the README file.

[dzhus]

@pbv Did you find an issue or was the PR closed by accident?

[pbv]

Sorry, my mistake

[dzhus]

@pbv This has now been pushed to Hackage as 0.2