Bug Fix: substitute spaces by dashes regex to support relative path

The previous `let word = substitute(word, '[ /]', '-', 'g')` command was
undesirably modifying relative links to have a '-' as the path
delimiter. For example 
[[Link description|path/to/link.markdown]] 
was being changed to 
[[Link description|path-to-link.markdown]] 
the substitute command now respects nested file paths.