(site-config symbol) -> value
Symbol - a quoted symbol, either 'bin-path or 'mod-path
Returns a value indicating the configuration for the given key, or #f if the key is not valid.
Examples:
>> (site-config 'bin-path)
:: "./bin"
>> (site-config 'mod-path)
:: (".")
>> (site-config 'base-path)
:: #f