Revise config.sh to not use hardcoded list of plugins. Also quote vars.

Previous plugin list was hardcoded, which makes it difficult to do
plugin development, as it requires third-party plugin developers to
know to modify this script.

Instead, get the plugin source list using find, locating directories
(symlinks permitted).

Spaces in plugin names are handled appropriately, assuming you're
using bash 3.2-ish.

Additionally, variables, echos, etc. are mostly quoted now to avoid
odd cases where a space in a name might be treated as a command
invocation.