[core] Rework core implementation

Experimental re-implementation of core functionality with the aim:
- Depend only on the Python Standard Library for core
- If modules are missing elsewhere, *never* throw
- Unit test *everything*
- Cleaner and more minimal implementation
- Better integration points for existing implementations (charts,
  braille, etc.)
- Full backwards-compatibility with existing module system (except where
  modules can be vastly simplified)

themes/firefox-dark-powerline.json

@@ -1,42 +0,0 @@
-{
-	"icons": [ "awesome-fonts" ],
-	"defaults": {
-		"separator-block-width": 0,
-		"warning": {
-			"fg": "#002b36",
-			"bg": "#b58900"
-		},
-		"critical": {
-			"fg": "#002b36",
-			"bg": "#dc322f"
-		}
-	},
-	"cycle": [
-		{ "fg": "#B1B1B3", "bg": "#474749" },
-		{ "fg": "#BEBEBE", "bg": "#323234" }
-	],
-	"dnf": {
-		"good": {
-			"fg": "#002b36",
-			"bg": "#859900"
-		}
-	},
-	"pacman": {
-		"good": {
-			"fg": "#002b36",
-			"bg": "#859900"
-		}
-	},
-    "pomodoro": {
-        "paused": {
-            "fg": "#002b36",
-            "bg": "#b58900"
-        },
-        "break": {
-            "fg": "#002b36",
-            "bg": "#859900"
-        }
-    }
-
-
-}