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.

We make the temp file for the archive inside the existing temp directory
for a slightly simplification of the code.

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 scheme is that imports go into three groups: stdlib, non-stdlib and project-local.
In this case the local project is github.com/juju/charmstore.

   go get github.com/rogpeppe/sortimports

gets you a tool that automatically sorts imports for you in this
way.