New -find feature allows searches in parent directories

[heaths]

If you were to call vswhere -find ..\**\whatever.exe, vswhere will happily walk the directory tree from the parent directory of instances. This isn't a security problem since the account running vswhere needs permissions to enumerate those directories as well as to read/execute any results, but is still something we should probably prevent in the initial offering to avoid an awkward change to block this unsupported case in the future.