Database configuration

To go with the great work on #12 by @christiansmith

This allows you to specify your database config any way you can any other config option.

```
wiki --database='{"type": "./redis"}'

wiki_database='{"type": "./leveldb"}' wiki

echo '{"database": {"type": "./redis", "port": 123, "host": "foo"}}' > config.json && wiki

echo '{"database": {"port": 123, "host": "foo"}}' > leveldb.json && wiki --config=leveldb.json
```
The only option that should be required for a database to work is "type", which is how the database is to be required. It is the only property with a default value, which is "./page".