Add if/else statements

[MysteryBlokHed]

Closes #83

Adds support for if/else statements. The keyword runif was used instead of if to avoid conflicting with the execute if command.

• Implement if/else statements
  • Add if statements
  • Add else statements
  • Make parse_shorthand run recursively until no if statements/while loops are left
  • Make sure that if statements cannot modify the result of the condition test
• Add tests
• Add documentation
  • Update syntax table
  • Add examples

[MysteryBlokHed]

Testing the output of code that generates if statements might be hard to do if there's more than one, but some code from the while loop tests could probably be reused.

[MysteryBlokHed]

A new issue (#85) has been added that will make easier testing of if/else statements and while loops possible.
Until then, I think it's alright to merge this, since nested/multiple if statements have been tested and the code likely won't be modified any time soon.