Add rejs cli for npm

Hi @Moncader

I added cli for npm. Its behavior is the following:

---
Print help message
```
$ rejs -h
Usage:
    rejs [options] [file ...]

Examples:
    rejs foo.js bar.js baz.js
    rejs --out out.js foo.js bar.js baz.js

Options:
  -h, --help     Print this message
  -o, --out      Output to single file
  -v, --version  Print rejs version
```

Print rejs version
```
$ rejs -v
v0.0.2
```

Output to stdout
```
$ rejs samples/node/[^node]*.js
var namespace = {};(function() {

  namespace.one = {
    hello: {}
  };

}());(function(global) {

  global.namespace.one.hello.two = 'Hello, world!';

}(this));
```
Output to single file
```
$ rejs -o samples/node/out.js samples/node/[^node]*.js
$ cat samples/node/out.js
var namespace = {};(function() {

  namespace.one = {
    hello: {}
  };

}());(function(global) {

  global.namespace.one.hello.two = 'Hello, world!';

}(this));
```
---

Could you please any thought on this please?
Thanks.