Adding command line argument to disable listing of all log streams

[Miksu82]

In case where there are many many log streams, iterating through all of them takes forever which makes the stream argument useless if exact stream name is known.

This PR adds a new command line parameter to indicate that the stream parameter is exact match and awslogs shouldn't iterate through all streams.

I also considered an option where first an exact match would be tried and if found use that instead of iterating. But that would be a breaking backward compatibility so I decided to use a flag.

[coveralls]

Coverage decreased (-0.2%) to 91.579% when pulling 3b0154e on Miksu82:master into 0886184 on jorgebastida:master.

[mschfh]

@jorgebastida Please consider merging this PR, is there anything else needed to move it forward?

[takehark-amzn]

My environment has 400K+ streams in a log group so it takes minutes to list all streams and start retrieving the events in a stream. @jorgebastida Please consider merging this PR.