Release v2.0.0 preparation

[suikan4github]

• [x] Check all issues are listed in CHANGELOG.md
• [x] Check doc/refman.pdf is updated
• [x] Check doc/refman.pdf is "v2.0.0" in the front page.
• [x] Check CHANGELOG.md has v2.0.0 entry
• [x] Check all entry in Unreleased is moved to v2.0.0

git checkout master
git merge release --no-ff
git push origin master

git tag
git tag v2.0.0
git tag
git push origin v2.0.0