Allow special characters in directory name, optimization

[marmotton]

The keys of self.files are url encoded. This leads to errors when the path of a file contains special characters as src_path is not url encoded.

Modifications:

• url encode the directory path
• generate self.files in the on_files() function instead of on_page_markdown() (this is only an optimization, not strictly needed)
• test (directory with special characters)