charmload: some refactoring to address review comments

We add the capability for charmload to take parameters from the same
config file that charmd uses, splitting the config reading code into a separate
package to enable that.

Various other changes as suggested in review.

Some tests will arrive in the next PR - for now, I've done
some verification on the command line.

The rest of to packages in the tree use github.com/juju/errgo can we do that here too?