Support running Onionspray with upstream OpenResty packages

Problem

Besides having installation scripts to setup upstream packages, Onionspray right now does not work with them, due to the following issues:

• Use a native OpenResty substitution filter (#31).
• Ensure to always use the project's nginx error_log (#32).

Tasks

• Support running Onionspray with upstream OpenResty packages.
• Update documentation in how install/use Tor and OpenResty upstream packages through these scripts.

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)