Support for setting preserving and setting permissions of files copied with grunt.file.copy

When copying a file with grunt.file.copy, permissions are not preserved.

In addition, no option is provided to explicitly set permissions of the copied file.

A configuration option for one or both of these would be useful, for instance, when copying a shell script that must be executable.

Start at the grunt.file.copy entry point and inspect how copied files are handled. Check existing copy-related tests before deciding how configuration should express preserving or setting permissions. Done means copied files can preserve permissions or receive explicitly configured permissions, including executable shell scripts, with tests covering the behavior.

Written by the indexing model from the issue text.