No output to `stdout` when output file flag is empty [zentures/sequence#13]

[leolee192]

@Leftium opened zentures/sequence#13 and commented

Steps to reproduce:

1. `go run sequence.go analyze -i input.txt`

Expected output:

%action% %object%
#1 log messages matched
# get http://example.com
2016/05/12 11:27:43 Analyzed 1 messages, found 1 unique patterns, 1 are new.

Actual output:

2016/05/12 11:27:43 Analyzed 1 messages, found 1 unique patterns, 1 are new.

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No further details from zentures/sequence#13