Feature: Add user defined columns

[Zweih]

• Users can now specify columns:
  show packages with name, version, and size:

yaylog --columns name,version,size

• --columns overrides the defaults columns and --add-columns extends the default columns.

• Constants have also been separated into their own package.

• The -v flag has been removed and can now be used either with --add-columns=version or columns=version.

• Columns are now determined in the config package instead of the display package to maintain separation of concerns.

• Documentation has been updated to reflect these changes.