New (fifth) parameter in boss_flash:add - Data::term()

Sometimes MessageType, Message and Title is not enough - in my case I need to show both title, translated message and something like "11/100" (success count/total count).  
So I added a new parameter - arbitrary data. By default its' value is 'undefined'.

It can be accessed in templates with code like this:
```
{% for flash in boss_flash %}
    {{ flash.data }}
{% endfor %}

This change should be 100% backward compatible
```
Merge pull request #416 from brigadier/patch-5

New (fifth) parameter in boss_flash:add - Data::term()