Implement FLOGF formatting

Note: This *requires* an argument after the format string:

```rust
FLOGF!(debug, "foo");
```

won't compile. I think that's okay, because in that case you should
just use FLOG.

An alternative is to make it skip the sprintf.

fish-rust/src/flog.rs

@@ -191,10 +191,9 @@ macro_rules! FLOG {
     };
 }
 
-// TODO implement.
 macro_rules! FLOGF {
-    ($category:ident, $($elem:expr),+ $(,)*) => {
+    ($category:ident, $fmt: expr, $($elem:expr),+ $(,)*) => {
-        crate::flog::FLOG!($category, $($elem),*);
+        crate::flog::FLOG!($category, sprintf!($fmt, $($elem),*));
     }
 }