ignore code coverage and api scrape output (#4412)

DevProject/.gitignore

 /Assets/Demonstrations*
 /Assets/ML-Agents/Timers*
 /csharp_timers.json
+/CodeCoverage/
 
 # Environemnt logfile
 *Project.log

com.unity.ml-agents.extensions/.gitignore

 build.sh.meta
 build.bat.meta
 .idea/
+
+# API Scraper output
+*.api
+*.api.meta

com.unity.ml-agents/.gitignore

 # Visual Studio 2015 cache directory
 /.vs/
 
+*.api
+*.api.meta