Add an option to copy task for symlinks

[SergeyMaltsev]

Recent change in Grunt v1.5.0 has broken our build process, since it copies symlinks as symlinks which is not expected.
Changing copied file contents after copy, changes original file because it is a symlink.

So in fact, it is not backward compatible with the introduced change.

Expected: Symlink should be copied as new file as it was in pre v1.5.0 version.

Please add an option to support symlink copying for grunt.copy task, but it should be false by default to not break existing functionality.