ci: add node-14.x to workflow (#101)

diff --git a/appveyor.yml b/appveyor.yml
index fb046ec..610ad08 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -23,6 +23,7 @@
   matrix:
     - nodejs_version: "10"
     - nodejs_version: "12"
+    - nodejs_version: "14"
 
 install:
   - ps: Install-Product node $env:nodejs_version