Deprecate -o/-O offset switches in favor of -b switches

[resilar]

Quoting myself from comment thread of issue #9 "Lazy -b evaluation"

I think we should consider deprecating offset switches -o/-O at some point because -b switches are now expressive and user-friendly enough to offer the same functionality: -o 42 and -b 42: are essentially identical. Similarly, -O 42 and -b -42: are equivalent to each other. The only disadvantage of -b command-line options is that appending to the input is unsupported when forging CRC checksums, whereas -o and -O do support appending to the end of the input message. However, the current append mechanism should probably be rewritten anyway to support prepending also (as discussed in issue #11).