This is a simple configuration file parser. A configuration file has the follwing structure:
# Comments can be placed after '#' character.
VARIABLE_1 VALUE_1 # VARIABLE_1='VALUE_1'
VARIABLE_2 : VALUE_2 # VARIABLE_2=' VALUE_2'
VARIABLE_3 : # VARIABLE_3=''
VARIABLE_4 1
VARIABLE_5 ${VARIABLE_${VARIABLE_4}} # VARIABLE_5='VALUE_1'