build: get rid of "new" target

Makefile

@@ -1,7 +1,7 @@
 GO = go
 BIN = bin/aoc
 
-.PHONY: build test clean new
+.PHONY: build test clean
 
 build:
 	@mkdir -p $(dir $(BIN))
@@ -13,12 +13,9 @@ test:
 clean:
 	@rm -f $(BIN)
 
-new:
-	@if [ -z "$(filter-out $@,$(MAKECMDGOALS))" ]; then \
-		echo "Usage: make new 2020D9"; \
-		exit 1; \
-	fi
-	@DAY_ARG=$$(echo $(filter-out $@,$(MAKECMDGOALS)) | awk '{print $$1}'); \
+%:
+	@DAY_ARG=$@; \
+	if echo $$DAY_ARG | grep -qE '^[0-9]{4}D[0-9]+$$'; then \
 		YEAR=$$(echo $$DAY_ARG | sed 's/D.*//'); \
 		DAY_NUM=$$(echo $$DAY_ARG | sed 's/.*D//'); \
 		DAY_NAME=$$(case $$DAY_NUM in \
@@ -60,6 +57,7 @@ new:
 		else \
 			touch internal/data/$$YEAR/Day$$DAY_NAME/input.txt; \
 		fi; \
-
-%:
-	@:
+		echo "$$DAY_ARG ready to be solved."; \
+	else \
+		:; \
+	fi