provide object storage with lifetimes for config/items/scopes

[RonnyPfannschmidt]

in various internal and external plugins we find outselfes attaching objects to either items or config as pseudo-private elements sometimes even going as far as creating plugin instances only as means for storage

i'd like to open an api to provide storage to plugins that eases getting a own namespace and controlling for how long something will be stored

[pytestbot]

GitMate.io thinks possibly related issues are #162 (Store terminal width on config object), #1373 (provide a rich collection object in the pytest_modifyitems hook), #1137 (xdist: boxed send an item object to a subprocess), #1649 (unittest instance objects exist for the lifetime of the py.test run), and #1743 (config.fromdictargs applies the dictionary after the config object construction instead of ).