Feature mulit langs for workshopper


![mulit-langs-workshopper](https://cloud.githubusercontent.com/assets/27926/2824454/36443c26-cf32-11e3-964d-cf7f76a14d7a.gif)


About initial target language, it will check `process.env.LANG`,  and match that `exercises*`.
If match, it will be stored in `~/.config/learnyounode/workshopper.json `,
else `exercises/` will be becomed default lang. 

Currently, I just only tested it in Mac. I need more testing. :)

## Details:   

### Directory Structure
```
exercises/       
exercises_zh-CN/
exercises_fr/
...
```
### An Exercises structure
```
exercises
├── help.txt
├── locale.js
├── menu.json
...
```

### Every Exercises dir has a `locale.js` file.
```js
const path = require('path')

function fpath (f) {
  return path.join(__dirname, f)
}

module.exports = {
    name        : 'learnyounode'
  , menu        : require('./menu')
  , title       : 'LEARN YOU THE NODE.JS FOR MUCH WIN!'
  , subtitle    : '\x1b[23mSelect an exercise and hit \x1b[3mEnter\x1b[23m to begin'
  , helpFile    : fpath('help.txt')
}

```

### Storing the `lang`
```
~/.config/learnyounode/workshopper.json  
```

## Test case with learnyounode
```js
multLangsWorkshopper({
    name        : name
  , appDir      : __dirname
  , menuItems   : [ {
        name    : 'credits'
      , handler : credits
    } ]
})
```

Related https://github.com/rvagg/workshopper/issues/48