v0.8.0

What's Changed

• chore!: remove useless .isValid()
• chore!: now .add() and .subtract() return nullable
• feat: add method addRound to prevent date overflow by @g1eny0ung in #41
• feat: add method subtractRound to prevent date overflow by @g1eny0ung in #42

Full Changelog: v0.7.2...v0.8.0