Added proxy configuration to the HTTPClient

Hi, this is a first attempt to use a proxy with the plugin. It works for basic http-proxies without user authentication.
@methodus I realize this is a long time ago but anyhow. I can't figure out why I would exclude _http_ proxies here:

    [...]
    && !proxy.name.startsWith("http"))
    [...]

I figure you want to exclude https proxies since the scheme you're adding below is a pure HTTP scheme?! Shouldn't this be changed to 

    && proxy.name.startsWith("http:")

instead? This is related to #41.