add comments to explain structure of program line objects
prepare to implement INPUT statements
save changes before branching to implement INPUT statements
prepare for implementing input prompt
remove _prompt version of <string_input> and <numeric_input> tags
change @console_input to @input
make reference to KeyTalker available to Input Helper
added code to print input prompt, but doesn't work right
correctly reads input, but doesn't do anything with it
only run program statments while mode = normal
accepts input, then continues program
get rid of 'input_status', retain 'input_mode'
change name of InputHelper class to UserInputHelper
change name of ProgramRunner class to StatementRunner
change tag for StatementRunner object to @statement_runner
change tag for ProgramController object to @program_control
INPUT statement correctly sets user-entered value for string variables