Add the ability to use path in --plugin-load and --plugin-load-add options

Sometimes it would be good to have the ability to load plugins from different directories. Currently it can be done only from the directory set in "plugin_dir" system variable. But if we want, for example, to develop mtr test which would use plugins located in different locations in build directory(see EXAMPLE_PLUGIN and PLUGIN_AUTH in plugin.defs), the solution where it's able to load plugin only from one directory doesn't work. --plugin-load and --plugin-load-add are just command-line options which even don't have the corresponding system variables, they can't be replicated as well as changed with query, so it's would be quite safe to load some plugins from custom directories with these options.