Added server variables support, added multiple servers support

[romalytvynenko]

closes #72

Maybe (depends if I add multiple servers support to config)
closes #50

• Server variables config (server factory?)
• Proper alternative services propagation (operation level should be aware about path level which should be aware about root level).
• How to inject more data/objects into extensions?
• Tests
  • Alternative server is added to operation when no matching server found
  • Alternative server is not added when there is matching server
  • Alternative server is added there is matching and not matching servers
  • Alternative server is moved/not moved to the path when all path operations have/don't have it
• Docs