core, github, kobo updates

core:

• support '' for "empty paths"
• helper for computing stats, add to more modules

github:

• deprecated my.coding.github, see ca39187
• support data arbitration from GDPR and ghexport, see d7aff1b

kobo: see #62

• breaking: kobuddy should be properly installed (instead of old my.config.repos.kobuddy)
• breaking: export_dir renamed to export_path for consistency,