Make conn values and services path configurable at runtime

Connection values (such as redis server port and the port to run farside
on) as well as the services json file to use can now be set via
environment variables:

FARSIDE_PORT sets the port for Farside to run on
FARSIDE_REDIS_PORT sets the redis server port for Farside to use
FARSIDE_SERVICES_JSON sets the services json file for Farside to use

This partially addresses the move towards de-listing Cloudflare
instances by default by allowing different services json files to be
used with different redis servers.

See #43

README.md

@@ -63,8 +63,8 @@ Farside's routing is very minimal, with only the following routes:
     particular service with the specified path
   - Ex: `/libreddit/r/popular` would navigate to `<libreddit instance
     URL>/r/popular`
-    - If the service provided is actually a URL to a "parent" service 
-      (i.e. "youtube.com" instead of "piped" or "invidious"), Farside 
+    - If the service provided is actually a URL to a "parent" service
+      (i.e. "youtube.com" instead of "piped" or "invidious"), Farside
       will determine the correct frontend to use for the specified URL.
   - Note that a path is not required. `/libreddit` for example will still
     redirect the user to a working libreddit instance
@@ -108,3 +108,6 @@ request per second per IP.
 | Name | Purpose |
 | -- | -- |
 | FARSIDE_TEST | If enabled, bypasses the instance availability check and adds all instances to the pool. |
+| FARSIDE_PORT | The port to run Farside on (default: `4001`) |
+| FARSIDE_REDIS_PORT | The Redis server port to use (default: `6379`, same as the default for Redis) |
+| FARSIDE_SERVICES_JSON | The "services" JSON file to use for selecting instances (default: `services.json`) |