add support for selective dilation

[dvolgyes]

Description

It would be very nice to replace the current dilation options
with a more intelligent one.
Currently the number of dilations can be specified,
and the dilations always increase.
There are multiple problems with this, but the two largest ones:

• you cannot set the dilation
• you cannot set the order

So instead of the current integer input it would be nice to accept two kind of inputs:

• integer, which has the same meaning as currently
• tuple, where the length of the tuple means the number of dilation steps,
  and the numbers represent the pixel distance / dilation.
  E.g. 1,2,4,1 means 4 blocks, with dilation of 1,2,4 and 1.

There are good arguments that a final refinement could be very helpful
to remove small scale noise. Also, the receptive field is better tunable
this way.
The implementation is relatively straightforward.

[fepegar]

Hi, @dvolgyes. Sounds good. Would you like to contribute with a PR?