Add separation attribute (or force radius attribute)

[Ary Pablo Batista]

Sometimes the circular container get's a very large radius and elements become too separated from each other. There are two options to solve this feature:

1. To have a separation attribute capable of configure the circumference distance between elements
2. To have a _force_radius attribute to set the desired radius no matter the element position (works well if force expand is enabled).

This would be a really useful feature.