v2.0.0

Merge pull request #14 from Frezyx/dev

Migrate to null-safety, package update to v2.0.0

V1.2.0

Merge pull request #9 from Frezyx/dev

Add width and height fields for buttons

V1.1.0

• Added ability to set initially selected buttons
• Create travis & codemagic QA
• Fixed dart analyzer issues, errors and remarks

V1.0.0

• Basic functionality for creating a button group
• Two models of the group were created:
  • Radio (one selected button)
  • CheckBox (several selected buttons)