Enable Variable Substitutions for Windows

It would be nice to allow variable substitutions for windows machines as well.
Since the '$' is a valid character in windows paths, and these paths use the backslash (`\`) character instead of the forward slash, the default substitution cannot be used. Instead, I would propose the `%VARIABLE_NAME%` syntax.

Note that I am not a windows user, but a user of this library and it would be nice to offer support for windows as well.

I'm happy to discuss any implementation details and can also offer to work towards enabling this feature.

Start by reviewing how the library currently parses variable substitutions and handles path separators. The requested behavior is support for Windows paths using %VARIABLE_NAME% syntax while preserving existing substitutions; done means the syntax works correctly on Windows without breaking current behavior.

Written by the indexing model from the issue text.