Add multiple middlewares support

[antoinegomez]

This resolves #15

In some projects I need to secure some assets and I had to create a new koa instance. That's worked but I find it easier to allow to pass multiple middlewares instead of one.

Tests supplied and documentation updated.
As I work with koa@2 I did not updated the code for the stable release but it could be easily done if requested by somebody else.

[haoxins]

Personally, I use koa-router for these cases.

[antoinegomez]

Not thought about that way, I guess it's something like that:

router.get('/secure', isAuthedMiddleware, serve('./secure_files'));

Nonetheless I think it does not harm and could help.

[haoxins]

@antoinegomez agreed, I just misunderstood something! 😢

But it's better to pass multiple middlewares as separate arguments, such as

app.use(mount('/secured', isAuthed, world))

// not

app.use(mount('/secured', [isAuthed, world]))

[haoxins]

@antoinegomez I just switch next branch to master.

Please submit this PR to master.