Add --ignore-minor-version

[otfried]

Add flag to allow reading files that were written with a newer version of Ipe. It should print a warning like this:

I am ignoring the minor file format version in the document. If the document actually uses any of the features introduced 
by a newer Ipe version, then reading it now may result in an error message during parsing, may lose some data, or may 
actually corrupt the document.