Various changes to the `Formatter`:
* Introduce `COLUMN_TEMPLATE_PADDED` and `COLUMN_TEMPLATE_UNPADDED` constants with their appropriated methods `get_padded_template()` and `get_unpadded_template()` to ease the use of column templates
* Remove the default value of the `spacing` parameter in `get_line_template`
* Remove last paragraph of `get_line_template()` docstring since the exact behavior is explained in `get_padded_template()`
* Introduce `_get_template()` in `FormatterTest`