Handle error when no argument is passed

Currently invoking `index.js` with no arguments throws the error:

``` js

/Users/brandondail/.dev/node-todo/index.js:10
const flag = args[0].charAt(0) === '-' ? args[0] : null
^
TypeError: Cannot read property 'charAt' of undefined
at Object. (/Users/brandondail/.dev/node-todo/index.js:10:21)
at Module._compile (module.js:460:26)
at Object.Module._extensions..js (module.js:478:10)
at Module.load (module.js:355:32)
at Function.Module._load (module.js:310:12)
at Function.Module.runMain (module.js:501:10)
at startup (node.js:129:16)
at node.js:814:3

```

Handle this and other argument-based errors gracefully

Start by reading index.js around line 10 and run the CLI with no arguments to reproduce the TypeError. Inspect the argument handling for other invalid or missing values; done means argument-based errors are handled gracefully without an uncaught exception.

Written by the indexing model from the issue text.