Add rustfmt nightly to appveyor install

Author: Michael Wright

appveyor.yml

@@ -23,6 +23,7 @@ install:
     - del rust-toolchain
     - cargo install rustup-toolchain-install-master --debug || echo "rustup-toolchain-install-master already installed"
     - rustup-toolchain-install-master %RUSTC_HASH% -f -n master
+    - rustup component add rustfmt --toolchain nightly
     - rustup default master
     - set PATH=%PATH%;C:\Users\appveyor\.rustup\toolchains\master\bin
     - rustc -V