Add non-Windows support

[soulweaver91]

• Detect an xdelta executable installed in the system (including not assuming "xdelta3.exe" by default)
• Write apply.sh
• Make it possible (but not a requirement) to bundle multiple, specified xdelta executables

[soulweaver91]

Multiple executables could probably be stuffed into the existing -x switch: use the first provided one to write patches, package all of the provided ones.