Add option for suppress logs in test cases.

It would awesome to have a functionality similar to the [ExUnit tags](https://hexdocs.pm/ex_unit/ExUnit.Case.html#module-log-capture). Especially for the suppression of the logs generated in the tests.

```
@tag :capture_log
it "should forward the error result and stop" do
#...
do
```

It would be useful for cases like this one, where ran three tests successfully but the first one is showing a ugly log.
```
.2017-10-11 14:46:57.783 nonode@nohost pid=<0.30.0> : Process #PID<0.576.0> raised an exception
** (RuntimeError) crash_test
spec/orders_service/fsm/recipient_spec.exs:683: anonymous fn/0 in (....)
..
```

This functionality already exists and is not documented correctly? or could be added in a near future?
A function `capture_log` already exists, but the purpose is different.