Fix unreleased commits section

[wadackel]

What does this do / why do we need it?

Unreleased section was added in keep-a-changelog template.

## [Unreleased]
### Features
- hello

## v0.0.3 - 2018-04-05
### Features
- saying goodbye

## v0.0.2 - 2018-04-05
### Bug Fixes
- corrected greeting

## v0.0.1 - 2018-04-05
### Features
- initial commit

Related #15

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And, update template format to human readable 📝

How this PR fixes the problem?

• None

What should your reviewer look out for in this PR?

• None

Check lists

• Test passed
• Coding style (indentation, etc)

Additional Comments (if any)

• None

Which issue(s) does this PR fix?

• No changelog for Unreleased version #15

[coveralls]

Pull Request Test Coverage Report for Build 78

• 155 of 172 (90.12%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+1.03%) to 72.147%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/git-chglog/custom_template_builder.go	49	51	96.08%
chglog.go	32	37	86.49%
cmd/git-chglog/kac_template_builder.go	74	84	88.1%

Files with Coverage Reduction	New Missed Lines	%
chglog.go	1	85.35%

Totals
Change from base Build 76:	1.03%
Covered Lines:	1435
Relevant Lines:	1989

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💛 - Coveralls