simplify attribute check, pre-selected values

Simplifies check when adding attributes, our attribute function returns
proper values when no attributes are specified, making the check on the
attributes array unnecessary. We can simply call the attributes function
all the time and not worry about it.

Add support for pre-selected values when creating selects/multi-selects.
Prepending "selected_" to the value string (the key in the options
array) pre-selects the option.

Include pre-selected examples in demo.php.