added simple variable for S for plural/singular units

Added scope variables for: daysS, hoursS, minutesS, and secondsS.

The variables are set to 's' or '' (blank string) depending on if each of the values should be plural. I thought of a bunch of different ways to implement this, but this seemed like the simplest. I thought about adding configurable unit words or a template variable you could provide, but it just seemed like overkill.

Here is an example of how it would be used:

```html
<timer class="countdown" end-time="campaign.start">{{days}} day{{daysS}}, {{hours}} hour{{hoursS}}, {{minutes}} minute{{minutesS}}, {{seconds}} second{{secondsS}}.</timer>
```

Which would result in (for example:
```
22 days, 3 hours, 2 minutes, 4 seconds.
22 days, 3 hours, 2 minutes, 3 seconds.
22 days, 3 hours, 2 minutes, 2 seconds.
22 days, 3 hours, 2 minutes, 1 second.
22 days, 3 hours, 2 minutes, 0 seconds.
22 days, 3 hours, 1 minute, 59 seconds.
...etc
```

Sorry that my description is longer than the code I submitted.