feat(utils, templates, docs): file-based templates + stronger README

templates/problem/test/main_test.go.tmpl

@@ -0,0 +1,12 @@
+// LeetCode {{NUMBER}} 單元測試（Go testing）
+package main
+
+import "testing"
+
+func TestExample(t *testing.T) {
+    // TODO: input := []int{}
+    // got := solve(input)
+    // want := 0
+    // if got != want { t.Fatalf("want %v got %v", want, got) }
+}
+