Add support to parse JSON array (JSON Parser and HTTPJson)

[johnrengelman]

Required for all PRs:

• [ X ] CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• [ X ] Sign CLA (if not already signed)
• [ X ] README.md updated (if adding a new plugin)

[johnrengelman]

Our use case here is calling a web endpoint that already parses some data by "environment" which is tag we'd like to put on these metrics. This allows the JSON parser to parse the same metric with variable tags.

[sparrc]

couldn't the parser just figure this out without a configuration option?

[johnrengelman]

Yeah, probably. I'll give that a shot.

[sparrc]

@johnrengelman you can use the bytes package to detect it with something like this:

func isarray(buf []byte) bool {
    ia := bytes.IndexByte(buf, '[')
    ib := bytes.IndexByte(buf, '{')
    if ia > -1 && ia < ib {
        return true
    } else {
        return false
    }
}

[johnrengelman]

@sparrc updated w/ your feedback. Good call on that. Must nicer. I guess I was initially thinking that explicit configuration would be appropriate, but I think just enforcing this convention works best since there's no other way to handle a root array.

[dgnorton]

Can you remove test.sh? I think the example will be a little clearer with just one command.

[johnrengelman]

Pushed an update for this.

[dgnorton]

Missing a quote.

[johnrengelman]

:sigh: and fixed.

dgnorton will finish review