Add support for IDE variant mappings that include subvariants

For example, in your Msvc generation hints, you can now write:
VariantMappings = {
  ['release-default']    = 'Release'
  ['release-analyze']    = 'Release-Analyze',
}
...and you'll get a Release and Release-Analyze configuration in your IDE.

One could already achieve this with the FullMapping hint, but that forces
one to enumerate all build configurations, which is tedious.
Add support for VS 2013 'baked' toolset