Load config from JSON file

[Turbo87]

This implements issue #927 and allows us to load Flask config from JSON files that can be shared for example with frontend dev tools like grunt.

[untitaker]

Isn't from_dict mostly the same as update?

[Turbo87]

@untitaker you're totally right... I missed that Config was derived from dict. I'll rework the commits later today to use update() instead.

[Turbo87]

@untitaker as expected, it seems to work well with update() too. I've pushed the new from_jsonfile() commit and removed the from_dict() commit.

[mattupstate]

@Turbo87 @untitaker This is a duplicate of #827

[Turbo87]

@mattupstate you're right, I didn't see the other PR when I had a quick look if one already exists. Is there any chance this will be merged any time soon?

[untitaker]

@Turbo87 pallets/jinja#283 (comment)