Add generic SOCKS proxy support, variable renaming

Now you can run your workers through a SOCKS proxy with the -S flag!
Authentication at the command-line through fully-qualified URLs as well
as interactive authentication are now supported.

I may revert the try/except block in LinodeProxy's constructor should it
be found to be non-advantageous in further testing.  Performance in
creating large quantities of Linode proxies is still dog slow, may use
concurrent.futures to speed this up in the future (hah) -- this will
require a major rework of the downloader main().

squid_dl/proxy.py

@@ -0,0 +1,38 @@
+"""
+squid_dl Proxy base class (for typing)
+
+              ⣀⣠⣴⣷⣾⣿⣿⣿⣶⣦⣠
+             ⣴⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡖
+           ⣰⣿⣿⠟⠉⣀⠄⠻⣿⣿⣿⣿⣿⣿⡟
+          ⣠⣿⣿⡯ ⠜⠁ ⢰⣿⣿⣿⣿⣿⡿⠋
+      ⢀⣠⣼⣿⣿⣿⣿⣯⡄⡀⣠⣤⡿⡿⢿⣿⠿⠋
+    ⣀⣶⣿⣿⣿⡿⠿⠿⠿⢿⢿⣿⣿⣿⣿⣿⣿⣶
+  ⣠⣿⣿⣿⣿⡿⠗⠁     ⠙⣿⣿⣿⣿⣿⣿⣷⡆
+⢀⣴⣿⣿⣿⣿⣿         ⢻⣿⣿⣿⣿⣿⣿⣿⣥⠄
+⣴⣿⣿⣿⣿⣿⡃         ⢨⣿⣿⣿⣿⣿⣿⣿⣿⡅
+⠼⣿⣿⣿⣿⠋          ⠈⣿⣿⣿⣿⣿⣿⣿⣿⣗⠄
+ ⠹⠛⠏⠃           ⠸⣿⣿⣿⣿⣿⣿⣿⣿⣿⡁
+                ⣨⣟⣿⣿⣿⣿⣿⡏⣿⣿⣗
+               ⢠⣾⢷⣿⣿⣿⣿⣿⣯⢿⣿⣿⡂
+              ⣠⣾⣟⣾⣿⣿⣿⣿⣿⣿⣸⣿⣿⣧⠄
+            ⢀⣴⣿⣿⣽⣿⣿⣿⣿⣿⣿⣿⣧⢿⣿⣿⣷⣵⣀
+            ⢾⣿⣻⣾⣿⣿⣿⣿⣿⣿⣿⣿⡟⠘⢿⣿⣿⣿⣷⠄
+           ⢀⣤⣷⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡗ ⠈⠘⠉⠋⠁
+         ⢀⣴⣿⣿⡟⠋⠉⠁⠁⠁⠉⠹⣻⣿⣿⣧⡀
+        ⣰⣿⣿⡟⠉         ⢻⣿⣿⡄
+      ⢠⣾⣿⣿⠯            ⣻⣿⣷⣆
+     ⢠⣿⣿⣿⠩             ⠙⣿⣿⣿⣦⡀
+    ⢠⣿⣿⣟⡷⡕             ⠈⢻⣿⣿⣿⣷⡀
+    ⢰⣿⣿⡳⡩⣇              ⢙⣿⣿⣿⣿⣞⡄
+    ⢸⣿⣿⢧⡲⣧               ⢺⣿⣿⣿⣿⡽⡆
+     ⠻⣿⣯⠛                 ⠘⠟⠿⠿⡙
+
+"""
+
+
+class Proxy:
+    proxy_url = None
+    exclusive = False
+
+    def cleanup(self) -> None:
+        pass