Make quelpa parse the optional plist argument

This feature enables simple extension of the `quelpa' function.  For now
it's implemented as a hard-coded keyword lookup that sets the
appropriate internal variables.
rename to "upgrade" which seems to be more clear than "update"
necessary changes for package upgrades
fix version query
add doc for upgrading
add upgrade-test, document self-upgrading
Merge pull request #14 from quelpa/update-feature

Implement the update feature