feat: Add no-floating-promises (#10)

BREAKING CHANGE: This adds the stricter rule for floating promises
and this may break some builds. To disable the rule, overwrite the
rule and set it to `false`.

tslint.json

@@ -39,6 +39,7 @@
         "singleline": "never",
         "esSpecCompliant": true
       }
-    ]
+    ],
+    "no-floating-promises"
   }
 }