Add link target to makefile
Allow specifying md2html executable
Use less variables in project makefile
Improve nav listing

- Improve reliability of nav list
- Use line range for nav links instead of single line
- Allow renaming the index page
Support hiding files from nav
Update _nav while respecting exising list order
Get page/link titles from headers in HTML content
Allow .html extension in links to be disabled
Let user manually specify nav list
Compute titles using subshell instead of files