Foliantcontrib.macros

- Do not fail preprocessors run if no replacement for a macro is found.

This case can occur if some undefined environment variable is passed to the macro.

- Do not rewrite source Markdown file if an error occurs.

- Add tag `<m>...</m>`.

- Switch from unnamed to named parameters.
- Macro name is now defined in the tag body instead of "name" option.

- Preserve param case.