Add flatten (1302 flatten 1) option/key

[MDX-Tom]

As XP12 disables global flatten option in the in-game settings page, and meanwhile tones of airports remain unoptimized, we need to manually add a lot of "1302 flatten 1" lines to the apt.dat file.
My idea is to use this official parse tool to add those flatten lines automatically, but this tool seems to lack the ability to parse the "flatten" option/key in the apt.dat file.
The requested feature is to add the ability to parse the "flatten" option/key so that other python scripts may be able to use this tool to switch airports' flatten option.