src/plugin.[ch]: Implement `plugin_write', a function to pass values to specific...

src/plugin.[ch]: Implement `plugin_write', a function to pass values to specific write plugins.

This function does none of the magic `plugin_dispatch_value' does. It simiply
takes a value and passes it to the requested plugin.

This function is intended to be used with the upcoming `filter chain' code,
which will allow to pass values to specific plugins only.