Add an option to replace default loaders

Right now it's not possible to replace a default loader (e.g. for `CsvLoader`) except by using hidden/private methods. For example:

```python
from dataknead import Knead
from my_csvloader import CsvLoader

Knead._loaders["csv"] = CsvLoader

# Convert a csv with semicolons to json
print(Knead("./cities-semicolon.csv", delimiter = ";"))
```