Env variables in yaml config

Add support for .env variables in the yaml config. Closes #20 (closed). Also apply some minor fixes: #22 (closed)

config/for-docker.yaml

@@ -27,7 +27,7 @@ Audio:
     # time length of one recording block in seconds, for example 1800
     segment_time: 1800
     # name of the block, for example "%Y-%m-%d_%H:%M:%S_%s"
-    strftime: "%Y-%m-%d_%H:%M:%S_%s"
+    strftime: "%Y-%m-%d_%H:%M:%S"
     # the file format, for example "flac" or "ogg"
     format: "flac"
     # where to store the recordings, if empty engine-recorder/audio/recordings/block is used