* 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()

* fix multiple readme file rendering and fix #1657

* remove unnecessary loop