Create a library and add various options

[cdupont]

• added library in cabal file
• added attribute "echo=Above|Below" to get a display of the code
• added tests

[cchalmers]

This is great, nice to see this getting some attention. Couple of small things:

• I personally prefer diagram to diagrams to mark code blocks
• I'd be nice if Above/Below didn't have to be capitalised.

Otherwise this looks good.

I don't know if you noticed there's another PR #1 (which I started in December?!, I need to start finishing off my PRs) that has extra features and was pretty much ready to merge, but depends on a branch of diagrams-builder that never got finished. It's well commented if you wanted to look at it for ideas.

[bergey]

@cchalmers: I advised @cdupont that last I heard you weren't planning to finish the diagrams-builder PR, and hence, #1 wasn't likely to get merged. I'm still happy to revisit it. Certainly we need some story for multiple Backends.

I'll try to get through this in the next week, but probably not before Friday.

[cdupont]

@cchalmers: I renamed diagram to diagrams because that matched better the name of the tool. There might be other platforms generating diagrams :)
Regarding the capitalization, I'll fix that.

[bergey]

Should probably be readMaybe e. Or maybe a custom parser to allow other capitalization.

[bergey]

@cdupont What's your use-case for the library? I'm happy to split the library part from the executable, it just hadn't occurred to me that anyone would want this.

[bergey]

Why a table? We don't currently use a table in the documentation, and I generally try to avoid tables for layout in HTML.

[bergey]

I cherry-picked most of the changes, while preserving the command-line parsing and the existing recognized classes.

In particular, I included these changes:

• build with latest versions of all dependencies
• split library from executable
• parse attrs for choice of echo location
• tests

Thank you for this PR.