base_folder parameter should be copied with strdup in hoedown_document_new()

[Fábio Colácio]

I kept running into a double-free error when using Scidown, because apparently the hoedown_document_new does not copy the entire base_folder string, but instead copies the pointer.

This leads to undefined behaviour, because:

1. If I change the contents of the base_folder string after calling hoedown_document_new (), Scidown will no longer have the correct directory.
2. If Scidown frees or modify the string when I still need it, my code will act in an undefined way.

This is the problematic line:

scidown/src/document.c

Line 3309 in 14a78de

 doc->base_folder = base_folder; 

I would suggest changing it to doc->base_folder = strdup (base_folder)

Also, I would suggest changing the base_folder parameter to be const char* to prevent the string from being corrupted (or at least throw a warning at compile-time).