Options to specify http_proxy and https_proxy explicitly

[Javran]

This is somewhat a followup to #94.

In our use case we need to make connections with different Managers, with some going through proxy (with http_proxy and/or https_proxy) and others don't.

Looking at current implementation, it's only in body of newTlsManager and that of newTlsManagerWith that those two variables are dealt with, and helpers like parseSocksSettings are not exposed.

One workaround (I haven't tested this yet) I can think of is to set/unset environment variables as desired before initializing different managers, but I'd appreciate it if this can be done without doing IO mutations.