IDMLPackage.import_xml(): Manage ignorecontent and forcecontent flags.

simpleidml-ignorecontent='true' will skip the import of the xml content for the node and its children.
Except for the children which have the positive flag simpleidml-forcecontent.
README: simpleidml-ignorecontent and simpleidml-forcecontent tags explained.
README: better syntax highlighting.
README: use double backquote instead of simple ones.
Bump to v-0.91.6.
Store simpleidml-* attribute name into variables.
Merge branch 'master' into import-xml-updatecontent-flag
Fix the forcecontent flag for nodes that aren't direct children of a ignorecontent context.

Fix image paths in some of the XML test files.
Merge branch import-xml-updatecontent-flag into master.