Allow setting the plugin dir at runtime
This enables the use of bitlbee plugins in scenarios where there is no write access to the bitlbee lib/ directory.
One example is the NixOS linux distribution (which I'm currently packaging a bitlbee plugin for), where post-installation modification of a package (e.g. bitlbee) by another package (e.g. bitlbee-facebook) is not possible.
Another example would be a user without root access building and using a plugin with a system-provided (i.e. installed by root) bitlbee.
