initial appveyor config

[chcg]

see build results at https://ci.appveyor.com/project/chcg/notepad-pp-linter

• post build scripts are not working as expected except of win32 release build
• seems there is no reason to exclude win xp users and therefore used toolset v141_xp (VS2017)
• could be also adapted to build for v140/v140_xp (VS2015)
• could be also used for automatic github release deployment on creating tags, see commented config at the end and https://www.appveyor.com/docs/deployment/github/#provider-settings