v0.8.0

Thank you for all the contributions!

Fixes

• Fix button style defenition and add test for that #873 by @sryoya
• Convert Header block correctly #887 by @takanabe
• Add missing Image512 field in UserProfile #875 by @p1ass

Features

• Socket Mode Support #885 by @mumoshu
• Adding apps.uninstall method #879 by @gsavit
• Added timepicker form block element #870 by @jc21
• Allow message unfurling with auth URL #882 by @zerok

Misc

• remove .gometalinter.json #869 by @suzuki-shunsuke