Table cell converters

This PR adds support for *table cell converters*, converters that are not directly associated to a specific column. Instead a converter will check every cell value (except of those cells that belong to a column that has a particular converter), and if the value matches, the converter will do its conversion.

Configuration:

    processors:

    - !prepareTable
      converters:
      - !urlImage
          urlExtractor: <img src="([^"]+)".*
          urlGroup: 1