Stream management

Hi,

Currently the stream expression is (obviously) an expression so it takes lot of times to search and match against all existing streams.
It would be great to have a way specifying that we don't want a "search and match" but only "get me that stream".

Here is a time comparaison between `awslogs` and `aws logs` to get the same thing

```
admin@xxxxxxx:~$ time /home/admin/.local/bin/awslogs get /aws/transfer/s-047f907xxxxxxxxxxx global-dva-ci.0258452002e64965
[...]

real 0m47.783s
user 0m6.016s
sys 0m0.244s
```

```
admin@xxxxxxx:~$ time aws logs get-log-events --log-group-name "/aws/transfer/s-047f907xxxxxxxxxxx" --log-stream-name "global-dva-ci.0258452002e64965" --query 'events[].[message]' --output text
[...]

real 0m0.920s
user 0m0.548s
sys 0m0.064s
```

I could use `aws logs` in that case of course but `awslogs` is better for printing human readable timestamp.