Feature Request: Decorator for adding Trace names

As I'm marking up my functions for tracing, I find my self often doing this:

```python
def function_of_app():
  with context.iopipe.mark('function_name'):
     # function code
```

It would be very nice to be able to do something like this:
```python
@iopipe.mark('function_name')
def function_of_app():
   # function code
```