Please support HTTP proxy settings

[dejayc]

codesandbox doesn't seem to support HTTP proxies. For those of us behind corporate networks, this can be painful.

Ideally, codesandbox would look at the following environment variables in order to determine proxy settings:

FTP_PROXY
HTTP_PROXY
HTTPS_PROXY
ftp_proxy
http_proxy
https_proxy
no_proxy
NODE_TLS_REJECT_UNAUTHORIZED

Additionally, command-line parameters to control proxy behavior would also be useful.