Allow configuration of httpbingo env-vars/options

[plumdog]

Httpbingo has some configuration options: https://github.com/mccutchen/go-httpbin#configuration

It would be useful to be able to set these from the chart's values.

[redzioch]

@estahn Any update on this? I need such feature too and I would be superb to merge this PR and use your version of a chart, instead of having another fork.