config context hierarchy

[Jaymon]

as the config gets buried into with statements it should keep a history of all the contexts it has switched to and I should make sure it moves back to them in order:

with config.context("foo"):
    with config.context("bar"):
         with config.context("che"):
              # should be in che context here
         # should be in bar context here
    #should be in foo context here
# should be in "" context here

This might already work, but I should have a test making sure it works