Add plural handlebars helper

Closes #3414

- Adds a helper which shows a select string deopending on the number of items provided

```
{{plural pagination.total empty='No posts' singular='% post' plural='% posts'}}
```
If `pagination.total` == 0, output 'No Posts'
If `pagination.total` == 1, outputs `1 Post`
If `pagination.total` ==  2 or more, outputs `2 Posts`

Credit to @sebgie for making this work.
Merge pull request #3415 from PaulAdamDavis/plural-helper

Plural handlebars helper