Add some pattern to allow preprocessing parameter value on change

In some cases you want to be able to define a ``@param.depends`` decorator which watches a parameter and then modifies it. This currently leads to recursion, it may therefore be nice if we had a ``recursion`` flag, e.g. to allow doing things like:

```
@param.depends('polys', watch=True, recursion=False)
def _init_polys(self):
self.polys = self.polys.options(color='red')
```

Alternative suggestions to achieve the same thing would be welcome.