book: avoid filenames with colons and/or question marks

We just fixed the checkout on Windows by renaming files whose names
contain colons or question marks. Let's prevent those filenames (which
are invalid on Windows) from being re-generated in the future.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

script/book.rb

@@ -374,7 +374,7 @@ def next_section_url
   def save
     return if self.slug.nil?
 
-    path = self.absolute_path(self.slug)
+    path = self.absolute_path(self.slug.gsub(/[:?]/, ''))
     FileUtils.mkdir_p(File.dirname(path))
     File.open("#{path}.html", 'w') do |file|
       file.write(@chapter.wrap_front_matter(self.front_matter))