Creating a URL with invalid characters should raise an error

Currently creating a Data URL with data that contains invalid characters will successfully return a DataURL object. It should not do this and instead either return `None` or raise an error.

```python
from data_url import DataURL
url = DataURL.from_data("text/plain", False, "Hello World!")
print(url.url)  # Prints "data:text/plain,Hello World!", not a valid data URL!
```

Related to PR #16