Add `line_element_type` and `extraction_mode` to user config

Updating the user configuration with two keys:

• line_element_type. PyLaia is applied to text_lines by default. I want to be able to run it on half_subject_lines, so I'm adding line_element_type to the user configuration.
• extraction_mode. PyLaia is applied to boundingRect by default. For Hugin Munin, I need to compare several extraction modes, such as polygon, min_area_rect...

Tested on Filae-us (process) with min_area_rect extraction mode on half_subject_line.