Add jsonian-format-region

[iwahbe]

This PR adds jsonian-format-region, with jsonian--huge-edit drawn from #48. To merge, jsonian-format-region must

• Be fast: we expect to beat json-pretty-print on large files. 4d27a2a already meets this requirement.
• Be accurate: we correctly format all json that jsonian-format-region is run on. We should produce a result at least as good as json-pretty-print.
• Tolerate comments: comments should persist through formatting when in jsonian-c-mode, each getting their own line.
• Be local: Allow formatting an arbitrary region in a larger json structure.
• Report progress: self-explanatory.
• Be documented: This function should be mentioned in the package header and in the README.md.

Of course all of these properties need to be under test.

To consider:

• provide a binding in jsonian-mode-map.

[iwahbe]

Huge edit implementations were borrowed from @taku0.