.mlrrc feature, and fix Windows build

• You can now save common defaults in a ~/.mlrrc. For example, if you normally process CSV files, you can say that in your ~/.mlrrc and you can leave off the --csv flag from your mlr commands. You can read more about this feature here, or in man mlr, or in mlr --help. This feature was requested in #339.
• The AppVeyor build is now unbroken and as a result there are Windows artifacts for this build. Sorry about the delay!! :^/