Release v0.18.0

[troosan]

• Wait that all tickets assigned to Milestone v0.18.0 are closed.
• Create the milestone & the release ticket.
• Set date of the release in CHANGELOG.md.
• Set version in docs/conf.py.
• Create and merge PR from correspondent branch to master.
• Close the milestone.
• Publish the release.
• Tweet the release @PHPOffice.

[michaljusiega]

Releated issue: #1979