enclose option should be an array rather than string (CLI)

Now we must use enclose option like this (CLI):  
`uglifyjs foo.js -c --enclose window:window --enclose document:document`

It's better to use enclose option like this (CLI):  
`uglifyjs foo.js -c --enclose window:window document:document`

Or like compress option style (CLI):  
`uglifyjs foo.js -c --enclose window=window,document=document`
