Added For Loops, REPL Help

Added for loops of the form
```
for i = 0, i < n, 1.0 in
  foo(n);
```
Which is basically START,GUARD,STEP.

There is also some help available in the REPL by using similar commands
as Haskell:

:h -- to show the help (also help)
:q -- to quit the repl (also quit)
:reset -- clear the current AST and start with a new one.