Add proxy support for both requests and curl mode

It's now possible to specify which proxy should be used for each account.
Requests mode still needs [1] to be fixed.

[1]: https://github.com/shazow/urllib3/issues/170

i.e.,
"accounts": {
  "GitHub": {
    "base_uri": "https://api.github.com",
    "https_proxy": "http://localhost:3128"
    ...
  }
}
...