Add linter command to makefile

Author: ShadowMitia

Makefile

@@ -1,8 +1,12 @@
 all: straight-to-web.xpi
 
 straight-to-web.xpi: *.js manifest.json
-	zip -r -FS ./straight-to-web.xpi * --exclude '*.git*' --exclude '*~' --exclude '.*' --exclude 'Makefile' --exclude 'README'
+	zip -r -FS ./straight-to-web.xpi * --exclude '*.git*' --exclude '*~' --exclude '.*' --exclude 'Makefile' --exclude '*.md'
 
+.PHONY: lint
+
+lint:
+	web-ext lint
 
 .PHONY: clean
 clean: