Various improvements

[djkaty]

These should probably be separate PRs but I figured they were small enough to place them together:

• Add Editor folder to .gitignore so that if it's currently in use by extract.sh it doesn't appear in source control
• Added 10 Unity versions to build_all.bat, extract.sh and notes.md + their headers
• Added a warning message to extract.sh if a particular installer can't be found when extracting, in any case any downloads are missed
• Output il2cpp-api-functions.h for every version in the api folder, and add all current versions to the repo
• Added a simple update.bat script that extracts any new versions, builds new headers and diffs for those headers plus the API headers

Any questions or problems let me know :)