Cannot pass look_for_keys via FS URL

While it is possible to pass `look_for_keys` to the underlying paramiko Client as follows

```python
from fs.sshfs import SSHFS

my_fs = SSHFS(..., look_for_keys=False)
```

I am unable to do so when using an FS URL:

```python
import fs

my_fs = fs.open_fs("ssh://[user[:password]@]host[:port]/[directory]?look-for-keys=False")
my_fs = fs.open_fs("ssh://[user[:password]@]host[:port]/[directory]?look_for_keys=False")
```

Looking at `SSHOpener`, this could be remedied by passing `look_for_keys` in when constructing SSHFS, much like e.g. `pkey` is.