Clean/reorganize gulpfile a little bit
fix "'return' outside of function" error
Faster script uglify (400ms -> 250ms)
Feed task require pages cache ?

I’m getting

```
TypeError:
/Users/MoOx/Sync/Development/putaindecode/website/mixins/feedPosts.jade:
10
    8|       description
    9|         | <![CDATA[[
  > 10|         | !{getFeedDescription(post.slug)}
    11|         | ]]>
    12|       pubDate #{(new Date(post.date)).toUTCString()}
    13|       <link>#{lang.baseURL}/#{post.slug}.html</link>

Cannot call method 'replace' of undefined
Please report this to https://github.com/chjj/marked.
```

when pages is not runned before
Make plumber optional

`--production` or `--plumber=0` will disable plumber now.
Maybe we should consider a --noplumber option instead ?