Add support for tarsnap options -H and -L

A new option `explicitSymlinks` will set `-H` when creating an archive.
This option makes tarsnap follow any symlinks specified explicitly on
the commandline, but not any found inside the file tree.

A new option `followSymlinks` will set `-L` when creating an archive.
This option makes tarsnap follow any symlinks found anywhere in the file
tree instead of storing them as-is.