Add jsobfu executable

This makes it a little easier to demo:

```
$ echo 'this.console.log("Hello")' | jsobfu 
this[(String.fromCharCode(99,0157,0156,0163,111,0x6c,101))][((function () { var R="g",T="o",t="l"; return t+T+R })())](String.fromCharCode(0x48,0145,0154,0154,0x6f));
```

#### Verification

- [x] `bundle install`
- [x] `echo 'this.console.log("Hello")' | jsobfu`
- [x] **verify** prints a reasonable obfuscated output to stdout
- [x] `echo 'this.console.log("Hello")' | jsobfu 4`
- [x] **verify** prints a much larger obfuscated output to stdout
- [x] `echo 'this.console.log("Hello")' | jsobfu | node`
- [x] **verify** prints `Hello` to stdout