Rules-based handling cells in table

[OlegShatin]

This feature should make maintenance process much easier by introducing new level of abstraction for tables-like sources of schedules. The general idea is to provide list of rules for GoogleSheet parser, where each rule will have own procedure to map cell to Schedule elems.

[OlegShatin]

Finally I've prepared proof of concept - every single cell is parsed by special rule. There are few things we need to finish before PR:

• Refactor the struct of cell rules on host side.
• Prepare tests for main types of cells.
• Check out the most complex cells, are they parsed properly or not.