Feature/spreadsheet unknown columns

[MartinKl]

An attribute can now be set for how to handle columns, that are not mentioned in the column map. Options are:

1. ignore (default, matches current behaviour, compatible with existing toml files)
2. fallback tokenization (String value)
3. only map to timeline tokens (empty namespace)

fixes #136

[barecheck]

Barecheck - Code coverage report

Total: 86.41%

Your code coverage diff: 0.03% ▴

Uncovered files and lines

File	Lines
src/importer/spreadsheet.rs	25, 33-35, 74, 90-97, 110, 126-133, 146, 162, 289, 405, 417-418, 427-428, 489-490, 499-500, 509-510, 519-520, 529-530