GitLab

* Change VS -> MinGW for CRAN

* Documentation to switch to MinGW by default

* Force cmake to run twice

* Try again dual build for Rtools

* Switch to cmake for building twice

* Try with Visual Studio as default, MinGW as fallback

* Try to remove VS appropriately when failing

* Attempt to get rid of build folder first

* Switch to unlink from rm rf

* Change lib_folder correctly when VS fails

* Add README updates from Microsoft/LightGBM@1c8355c

* Update default compiler doc and add GPU online install doc

* Better GPU doc