Ignore leading spaces in REPL

Fixes #1732.

Leading spaces in all commands are ignored before passing to the REPL parser - just like GHCi does.

```
Idris> True
True : Bool
Idris>   True
True : Bool
```

Neither does Idris REPL support `:{` and `:}`, nor it has a multiline mode like `:set +m`. Better to get rid of the mysterious `end of input` error at this moment: )