Implement sub apps

Users should be able to create separate instances of `microbe.js` apps and unite them based on a route. For example:

``` js

var adminApp = microbe()
var userApp = microbe()
var app = microbe()

app.use('/admin', adminApp)
app.use('/user', userApp)

```

No files or tests are named. Start by locating the app-construction and route-registration entry points, then trace how `app.use` handles mounted paths. Done means separate `microbe()` instances can be mounted under `/admin` and `/user` and route requests to the corresponding sub-app.

Written by the indexing model from the issue text.