Added time_ago filter translation possibility

Wordpress already offers a function for generating a translated time ago string: human_time_diff.
The only downside is, it just generates a string like "20 minutes", so I added some format variables.

For english you can still use {{ time | time_ago }}.
For a translation ( german here for example ):
```
{{ time | time_ago(format_past='vor %s', format_future='in %s') }}
```
This generates  ```vor 20 Minuten``` which is the german pendant to ```20 minutes ago```.

Fixes the issues 2) and 3) mentioned in https://github.com/jarednova/timber/issues/121