NEW: Add Config::moduleExists()

Directory-based check for whether a module exists in the manifest. Uses the same method of checking that the YAML `moduleexists: foo` does, just caches it and adds a way of accessing it through `Config`.

Usage:

```php
Config::inst()->moduleExists('sqlite3'); // boolean
```

Thinking: `class_exists()` is obviously the best way of doing this, but classes can be (and are) added/removed from some modules, so this can sometimes be a more robust check. Plus, the functionality to check already exists - it just can’t be accessed currently.
Thrice!