new init function to allow the user to do filehandling on her own.

[sezero]

This replaces the old PR #164 by @ChristianBreitwieser

[Ghabry]

I just quickly hacked together a VIO that reads from a zip: Works fine.

Only part I dislike is the documentation because it's not completely clear that the buffer shall contain the file content when returning.

This function should allocate a buffer which has the size of the requested file plus one (size+1) and fill the second parameter with the size of the file."

Maybe: This function should allocate and return a buffer which contains the content of the file and has a size of the requested file plus one (size+1). The second parameter is the size of the file, not of the buffer.

[psi29a]

Thanks for the review @Ghabry

[sezero]

Should we merge this?

[psi29a]

Looks good to me. :) Great work here.