Add support of if / else / end

[Flipez]

syntax should look like:

if(false)
  puts("if")
elif(5 < 4)
  puts("else if")
else
  puts("else")
end

[MarkusFreitag]

Should el replace else in general or just be added as an alias? Because replacing it would be a breaking change.

[Flipez]

Should be

if(false)
  puts("if")
elif(5 < 4)
  puts("else if")
else
  puts("else")
end

[Flipez]

Implemented by #198