Remove level file extensions.
Move level creation readme out of level dir.
Use stdbool instead of return code for load_level.
Add system level dir loading and level listing.
Update man page -l info.
Update usage message.
Update level dir, and add LEVEL_DIR override.
Fix level printing.

Return after failing to open the LEVEL_DIR to avoid causing a segfault.
Add ifndef guard around default LEVEL_DIR value.