Be able to change dicts/lists via CLI plugin

[ohinckel]

(copied from mknx/smarthome#165)

Currently only simple types (like str, int, ...) can be changed by the CLI plugin's up command. This patch add support for updating items with the type dict and list by using the following commands:

> ls test.dict
Items:
======
test.dict = {}
> up test.dict = { "key1" : "value1", "key2" : 2 }
> ls test.dict
Items:
======
test.dict = {'key2': 2, 'key1': 'value1'}
> 

> ls test.list
Items:
======
test.list = []
> up test.list = [ 1, 2, 3, "foo", "bar" ]
> ls test.list
Items:
======
test.list = [1, 2, 3, 'foo', 'bar']
> 

The implementation is using json.loads() method, which converts the string value and then try to update with the resulting value. So passing a wrong syntax or wrong structure type will end in the
Item test.dict: value ... does not match type dict. Via CLI ...
error.

Since it is implemented directly in the item's cast functions, all places updating the item values can now also use a string value for item of type dict/list. This could be a drawback and was not really inteted by this patch, but could on the other side be a handy feature.