Escape hatch for arbitrary methods

Example API usage:
```js
var Post = require('./post')
var Nanocache = require('nanocache')

var cache = new Nanocache()
cache(Post, 'prefetch', 1)
cache.render(Post, 1) // equivalent to cache(Post, 'render', 1)
```

This would make the API a bit more flexible while ensuring that `.render` stays easy to call in a clean way.

Start by reading the API entry points shown in the example: the callable cache(Post, method, args) form and cache.render(Post, args). Trace how method names are dispatched, then add coverage showing that cache.render(Post, 1) behaves like cache(Post, 'render', 1) and that arbitrary methods such as prefetch remain callable.

Written by the indexing model from the issue text.