Preprocessor and Postprocessor changes

- Ensure post processor for absolute paths does not add `file://` prefix.
- Add general base64 conversion post processor for all parsers.
- Add preprocessor to strip out multimarkdown's critic marks. Configurable to accept the proposed critic modifications or reject them (comments and marks are removed in both cases).
- Paths for images can be configured to: convert to base64, convert to absolute value or do nothing.
- Paths for non-image files can be configured to:  convert to absolute value or do nothing.
- Use codecs to unify file reading and writing for ST2 and ST3