[fstab] Write configurable options to crypttab (default: luks).

fstab.conf: Add a new "crypttabOptions" option that defaults to "luks".
            Document that for Debian and Debian-based distributions, the
            setting should be changed to "luks,keyscript=/bin/cat".

main.py: Append the options from the above setting to the end of every
         line in crypttab.

At least the "luks" option should always be there, because there may be
different encryption types. The Debian initramfs-tools also require the
Debian-specific keyscript option and will otherwise ignore the keyfile
entirely (see pull request #254).