Enhancement of the include directive

[ottawaguy81]

Can the include directive search set of paths provided via an environment variable. This will make things lot easier. Current design constrains the location of the include files.

I suggest :
export MARKDOWN_INCLUDE_PATH =x/y/z
markdown-pp <input.mdpp> -o <output.md>

I am not familiar with python, I don't know where to modify the code. I find the markdown-pp very handy. However it is limiting me in the auto generation of master markdown file from multiple small files due to the need for relative directory location.

[amyreese]

Happy to see this added, but will need someone else to contribute the changes.