Add --next-tag flag (EXPERIMENTAL)

[wadackel]

What does this do / why do we need it?

Added --next-tag flag to treat commits that have not yet tagged as specified tag names.
This is a function to include the commit for creating CHANGELOG in the release commit.

EXAMPLE:

$ git-chglog --next-tag 1.0.0

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?

• None

[coveralls]

Pull Request Test Coverage Report for Build 92

• 44 of 56 (78.57%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 72.353%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
chglog.go	43	48	89.58%
cmd/git-chglog/main.go	0	7	0.0%

Totals
Change from base Build 85:	0.2%
Covered Lines:	1476
Relevant Lines:	2040

---

💛 - Coveralls

[babarot]

Good job! Thank you

[wadackel]

@b4b4r07 Thank you, too 🐶