Fix source maps for development builds (#1772)

Let's have Browserify generate source maps again for development
builds, they make debugging the code a lot easier. But let's keep them
off for production builds, since the background.js bundle grows from
~2 MB to ~5 MB when we add them.

Makefile

@@ -4,6 +4,9 @@ ITEMS   := shared/html shared/data shared/img
 SASS = node_modules/.bin/sass
 BROWSERIFY_BIN = node_modules/.bin/browserify
 BROWSERIFY = $(BROWSERIFY_BIN) -t babelify
+ifeq ($(type),dev)
+	BROWSERIFY += -d
+endif
 KARMA = node_modules/.bin/karma
 
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