This is the only new thing (plus its test in `test_logging.py`). It's a simple context manager that sets and then un-sets `vispy`s logging level, so instead of:
```
> set_log_level('debug')
> function()
> set_log_level('info')
```
One could just do:
```
with use_log_level('debug'):
    function()
```
I would find this useful. And even if it isn't too useful for others, it's only a few lines so hopefully it's okay.