Add JSON InputSource to altsrc package

[johnweldon]

• Implement NewJSONSource* functions for returning an InputSource from
  various JSON data sources.
• Copy and modify YAML tests for the JSON InputSource

[johnweldon]

Is travis-ci not completing the build because the pull request is against the v2 branch?

[jszwedko]

Hmm, I don't see why Travis didn't get or respond to the webhook for this PR. I pulled and pushed your changes just now so that we can follow https://travis-ci.org/urfave/cli/builds/141945098 for build status.

Thank you for adding JSON support! Reviewing now.

[jszwedko]

This looks good! Did you consider just unmarhaling into a map to reuse the MapInputSource code (similar to what the YAML altsrc does)? Just wondering if there were technical limitations as it would DRY this up a bit.

Build is green: https://travis-ci.org/urfave/cli/builds/141945098

Thanks again!

[johnweldon]

@jszwedko thanks! Yes, I actually started with reusing the MapInputSource, but the json.Unmarshal doesn't work with map[interface{}]interface{}, only with map[string]interface{}

When I manually updated that top level to map[interface{}]interface{} after unmarshalling, I ran into type differences when using MapInputSource, I don't recall the specific errors, but for example, the json unmarshal treats all numbers as float64, which the MapInputSource couldn't handle when trying to get an int.

[jszwedko]

Ah, interesting, I do see that behavior regarding parsing all numbers as float64 which alone would make it difficult to use MapImportSource. Thank you for clarifying!

Will merge when the builds finish.

[anarcat]

i'm a little confused - this looks like it was merged, but the v2 README doesn't mention JSON support at all... where did that patch go?

[jszwedko]

Hey @anarcat, this was merged into the v2 branch, which is intended to be the precursor to a breaking 2.0 release, but I see that there really isn't anything breaking here, so I'll cherry pick to master as a separate PR.

You are correct that it still needs to be added to the README though 😄

[jszwedko]

Opened #598 to backport.

[anarcat]

well, the reason i'm confused is that the change to the README is in this PR (see 91563b8)

[jszwedko]

@anarcat indeed, I realized that when I went to port the feature to master. If you get a chance, would appreciate feedback on the PR (specifically if it works for your use cases!).