#85 [fit] finer control

[bordaigorl]

The fit library is really helpful in many situations.
It works as expected to create nodes that appear to include a bunch of other graphical objects.

But it can be used effectively in other scenarios: it is quite common to use the fit key in conjunction with shift to make a node as wide (high) as a set of other nodes and then displace it besides them.
I think use cases like this are sufficiently common to justify unpacking the fit key in smaller components to allow for finer control.

For example:

1. the current definition tries to fit all the anchors north, south, east and west; many times you only want the width to be asjusted; it would then make sense to also define fit width and fit height.
2. if you use the fit key more than once only the last will have an effect. It would be nice to have also fit variants to add some nodes to the fit key. This could work by just taking the max of the current size and the size covering the provided coordinates.