Refactor most of the code

- Replace panic()-based error handling in main() with
   Exception-based error handling
 - Put hardcoded defaults in config.di
 - Make general optimisations and readability improvements
   for a lot of the code
 - Change some of the ESVApi's interface

config.di

@@ -0,0 +1,18 @@
+/* default configuration for esv */
+
+module config;
+
+public:
+
+enum DEFAULT_APIKEY     = "abfb7456fa52ec4292c79e435890cfa3df14dc2b";
+enum DEFAULT_CONFIGPATH = "~/.config/esv.conf";
+enum DEFAULT_MPEGPLAYER = "mpg123";
+enum DEFAULT_PAGER      = "less";
+
+enum ENV_CONFIG = "ESV_CONFIG";
+enum ENV_PAGER  = "ESV_PAGER";
+enum ENV_PLAYER = "ESV_PLAYER";
+
+enum BUGREPORTURL = "https://codeberg.org/jtbx/esv/issues";
+
+// vi: ft=d