The -n command line option enables split mode for all proxyspecs,
effectively making sslproxy behave like sslsplit.
Divert option can be set/unset globally and per-proxyspec.
Add e2e tests for split mode, and update make file for tests
accordingly.
Update documentation accordingly.
Improve code reuse, remove duplicate functions.
This change deserves a release of its own, hence v0.8.4.
