Added exercises for 'Strings as Lists'.
Added exercises for 'Strings as Lists'.

chenleo/ipynotebook,BadWizard/intro_programming,nntisapeh/intro_programming,BadWizard/intro_programming,chenleo/ipynotebook,BadWizard/intro_programming,ehmatthes/intro_programming,ehmatthes/intro_programming,chenleo/ipynotebook,ehmatthes/intro_programming,nntisapeh/intro_programming,nntisapeh/intro_programming,nntisapeh/intro_programming,ehmatthes/intro_programming,chenleo/ipynotebook,BadWizard/intro_programming