fallback to default fonts when loading reader configuration

Since we are adding and renaming fonts, we need to make sure that
all fonts which reader uses are defined. This allows us to add fonts
and not break when users load old configuration files

close #252

reader.lua

@@ -124,7 +124,14 @@ Screen.native_rotation_mode = Screen.cur_rotation_mode
 G_reader_settings = DocSettings:open(".reader")
 fontmap = G_reader_settings:readSetting("fontmap")
 if fontmap ~= nil then
-	Font.fontmap = fontmap
+	-- we need to iterate over all fonts used in reader to support upgrade from older configuration
+	for name,path in pairs(fontmap) do
+		if Font.fontmap[name] then
+			Font.fontmap[name] = path
+		else
+			Debug("missing "..name.." in user configuration, using default font "..path)
+		end
+	end
 end
 
 -- set up the mode to manage files