Add support for MSVC 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 when the
number of configurations is high.
Merge pull request #221 from benharper123/master

Add support for MSVC IDE variant mappings that include subvariants