implement Target#set_default_facility wrapper

[goatshriek]

Stumpless provides the stumpless_set_default_facility function to allow a target to use a given facility for messages logged without a facility. This capability is currently missing from the Rust library wrapper, and needs to be added.

General Approach

There are a few details left out of the following approach, for you to fill in as you encounter them. If you find you need help, please ask here or on the project gitter and someone can help you get past the stumbling block.

The stumpless-sys crate includes a FFI binding for stumpless_set_default_facility already. You'll simply need to add a new function in src/target.rs to the Target trait. Name the new function set_default_facility, and use the other functions already in this trait as reference to implement the new function.