Add progress logging
[2015-10-10 01:13:56 +0900] fetch git tags
[2015-10-10 01:13:58 +0900] generate release note (v0.0.1...v0.0.2)
[2015-10-10 01:14:02 +0900] generate release note (v0.0.2...v0.0.3)
[2015-10-10 01:14:06 +0900] generate release note (v0.0.3...HEAD)