Niti love the command line

Nit is both a good compiled language and a good interpreted language (except that the naive interpreter is slow).

Some people already use nit as a traditional script-language by including a shebang at the first line of their programs and set them executable (`chmod +x`).

~~~nit
#!/usr/bin/env nit
print "Hello world"
~~~

This experimental PR make the nit interpreter more script-useful.

First, it introduces the `-e` option to run a program written on the command line.
Like with ruby, perl, bash and other script language.

~~~sh
$ nit -e 'print 5+5'
10
~~~

Second, and this is just wonderful, it add the `-n` option (from ruby and perl) to automatically iterate over the lines of files given as arguments. The current line is named `sys.line` (instead of `$_` in perl and ruby).

It works for stdin if no argument

~~~sh
$ echo "hello world" | nit -n -e 'print sys.line.capitalized'
Hello World
~~~

or on the arguments (as files) if one or more is given (it is the perl and ruby semantic)

~~~sh
$ nit -n -e 'print sys.line.capitalized' README
Nit Is A Statically Typed Object-Oriented Programming Language.
The Goal Of Nit Is To Propose A Statically Typed Programming Language Where Structure Is Not A Pain.
[...]
~~~

The logic of the `-n` is written in a library loaded at runtime (`niti_runtime.nit`), so can be updated without having to recompile the interpreter.