Support corp http proxies

[alexeagle]

Some users are in a corp network where all http traffic has to go through a proxy. bazelisk relies on http to talk to google storage API, github search API, and to download binaries.

It seems like at least support for the HTTP[S]_PROXY environment variables would be straightforward to add: https://golang.org/pkg/net/http/#ProxyFromEnvironment

If/when we have an npm package for bazelisk (#114) we should also read values from .npmrc to find the proxy config (https://docs.npmjs.com/using-npm/config#https-proxy) and pass that to the Go program. We can't tear down the @bazel/bazel npm package until this feature lands, because some users rely on the npm proxy setting to get Bazel onto their dev machines today.

[alexeagle]

Note, for the npm case, puppeteer is a good example of an npm program that wants to download additional binaries (headless chrome) and therefore has logic to find the proxy config (https://github.com/puppeteer/puppeteer/blob/0d243b7a0f4026b2f832abbdbca9633744209d16/install.js#L54-L64)

/cc @josephperrott

[coeuvre]

It seems Go's http package handle HTTP_PROXY/HTTPS_PROXY by default.

I tested on my Mac that bazelisk will use my HTTP_PROXY/HTTPS_PROXY values.

[alexeagle]

Oh that's great, I didn't test it myself so there is probably no issue here.