Add appveyor CI

Probably broken right now.

README.md

@@ -1,4 +1,4 @@
-# go-avif [![Build Status](https://travis-ci.org/Kagami/go-avif.svg?branch=master)](https://travis-ci.org/Kagami/go-avif) [![GoDoc](https://godoc.org/github.com/Kagami/go-avif?status.svg)](https://godoc.org/github.com/Kagami/go-avif)
+# go-avif [![Build Status](https://travis-ci.org/Kagami/go-avif.svg?branch=master)](https://travis-ci.org/Kagami/go-avif) [![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/github/Kagami/go-avif?branch=master&svg=true)](https://ci.appveyor.com/project/Kagami/go-avif) [![GoDoc](https://godoc.org/github.com/Kagami/go-avif?status.svg)](https://godoc.org/github.com/Kagami/go-avif)
 
 go-avif implements
 AVIF ([AV1 Still Image File Format](https://aomediacodec.github.io/av1-avif/))

appveyor.yml

@@ -0,0 +1,33 @@
+image: Visual Studio 2017
+configuration: Release
+
+clone_folder: C:\projects\go-avif\src\github.com\Kagami\go-avif
+
+install:
+  # Vars
+  - call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"
+  - set PATH=%PATH%;%LOCALAPPDATA%
+  - set GOPATH=C:\projects\go-avif
+  # Tools
+  - cd %LOCALAPPDATA%
+  - appveyor DownloadFile http://www.tortall.net/projects/yasm/releases/yasm-1.3.0-win64.exe -FileName yasm.exe
+  - appveyor DownloadFile https://github.com/mozilla/sccache/releases/download/0.2.8/sccache-0.2.8-x86_64-pc-windows-msvc.tar.gz -FileName sccache.tar.gz
+  - tar -xvf sccache.tar.gz --strip-components=1 --no-anchored sccache.exe && mv sccache.exe ccache.exe
+  # libaom
+  - git clone --depth=1 https://aomedia.googlesource.com/aom
+  - mkdir aom_build && cd aom_build
+  - cmake ../aom -G "Visual Studio 15 2017" -A x64 -DENABLE_CCACHE=1 -DCONFIG_AV1_DECODER=0 -DENABLE_DOCS=0 -DENABLE_EXAMPLES=0 -DENABLE_TOOLS=0 -DENABLE_TESTS=0 -DCONFIG_LOWBITDEPTH=1
+  - cmake --build .
+
+build_script:
+  - go get ./...
+
+artifacts:
+  - path: C:\projects\go-avif\bin\avif.exe
+    name: $(APPVEYOR_PROJECT_NAME)
+
+cache:
+  - '%LOCALAPPDATA%\Mozilla\sccache'
+
+on_finish:
+  - sccache -s