Allow specifying bar character through options object

Adds `options` object which allows user to override the bar character, like:

```js
require('time-grunt')(grunt, { barChar: ':' });
```

My use case for this was needing to use a US-ASCII character during deployment. I'm overriding it based on the `no-color` option, so I guess another approach would be for `time-grunt` to check that option directly, but it doesn't necessarily mean the output has to be US-ASCII only. `barChar` seems a bit clunky as well, open to suggestions.