* restructure markup & markdown to prepare for multiple markup languages support
* adjust some functions between markdown and markup
* fix tests
* improve the comments
* markup: microoptimise for many short filenames in directory
Move strings.ToLower() after the early-return length check. This is a safe operation in all cases and should slightly improve directory listing performance when a directory contains many thousands of files with short filenames.
* markup: expand test cases for IsReadmeFile()
