Option for load path appending

Currently Sagittarius only supports prepending however on some situation it is convenient to have appending option. e.g. load unsupported SRFI to write compatible library.

The option could be -A / --append-loadpath for appending load path. For dynamic library (I don't think this is needed but for consistency) could be -Y / --append-dynamic-loadpath (Y from the second letter of dynamic).