Support executable addin files

[directhex]

Since we are trapped in a world where mpack is zip, and zip as a format inherently has no support for permission handling, we ought to bypass this by adding an optional way to define an AddinFile as executable, such that when an addin file is installed, Mono.Addins can fix up the permissions post-hoc to allow execution.

This allows for shipping shell scripts, or other executable binaries, on Mac/Linux, inside mpack files.

Propsal:

[assembly: ImportAddinFile("scripts/myscript.sh")]

into

[assembly: ImportAddinFile("scripts/myscript.sh", true)]