Working modes

Server can work in the following modes:

1. record. Server proxies requests to the targetUrl and saves responses to file.
2. replayOrRecord. Server reads response from file. If there are no matching files, goes to targetUrl and saves response.
3. replay. Server reads response from file. If there are no matching files, 404 error.
4. replayOrProxy. Server reads response from file. If there are no matching files, go to targetUrl.
5. proxy. Server just proxies to the targetUrl

Mode is determined by:

1. mode param to mockProxy middleware
2. Using middleware mockProxyConfig (must be defined before mockProxy middleware)
3. Using environment variable KOA_MOCK_PROXY_MODE