Add OMNIBOT_CONFIG environment variable config

If you want to supply an OMNIBOT_CONFIG environment variable value, it's
available.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

omnibot/__main__.py

@@ -1,14 +1,15 @@
 import argparse
 import asyncio
 import logging
+import os
+from pathlib import Path
+import sys
 
 from .config import ServerConfig
 from .bot import Bot
 
 parser = argparse.ArgumentParser(description="Run an IRC bot")
-parser.add_argument(
-    "-c", "--config", type=argparse.FileType("r"), default="config.toml"
-)
+parser.add_argument("-c", "--config", type=str, default="config.toml")
 parser.add_argument(
     "--loglevel",
     type=str,
@@ -17,8 +18,15 @@ parser.add_argument(
 )
 args = parser.parse_args()
 
+config_path = Path(os.environ.get("OMNIBOT_CONFIG", args.config))
 config = ServerConfig()
-config.load(args.config)
+
+try:
+    with open(config_path) as fp:
+        config.load(fp)
+except FileNotFoundError:
+    print(f"ERROR: config file not found: {config_path}", file=sys.stderr)
+    sys.exit(1)
 
 # Set log level now
 loglevel = logging.INFO