document proxy.config.plugin.extensions_dir

Document extensions.config – allows a plugin to expose its symbols as "globals", thereby allowing it to expose it to others as API

Add an empty default config file, with coments depicting example usage.
Similar to plugin.config, extensions.config lists one plugin per line, with an additional boolean flag regarding how to open the .so file.

# Example
someplugin.so true

Would pass true as argv[1] to someplugin.so's plugin_init().