Filter fails with config or CSS file when path has spaces

[ryanmahan]

When the path to a .mermaid.css or .mermaid-config.json file contains a space, the command run does not include handling the space and fails.

Steps to reproduce:

1. Create a folder with a space in it
2. Create md file and CSS or JSON file in the folder
3. Run pandoc with the filter so it uses the CSS or JSON file

The -c /path/to/file/with space/.mermaid.css argument will cause the command to fail