Add remaining PSB2 problems

[IssaHanou]

Add the remaining problems from the list. For each problem a grammar should be created and an example program and test that the correct program can be found by Herb.

You can create a PR for each separate problem and check it on this list. You can also comment which one you are tackling so people don't work on the same one simultaneously.

See the paper for description of the problems

Problems:

• Bouncing Balls
• Bowling
• Camel Case
• Cut Vector
• Dice Game
• Find Pair
• Indices
• Leaders
• Luhn
• Mastermind
• Middle Character
• Paired Digits
• Shopping List
• Snow Day
• Solve Boolean
• Spin Words
• Square Digits
• Substitution Cipher
• Twitter
• Vector Distance