Support tree structure when adding branch to stack

[geofflamrock]

This PR is part of supporting a tree structure with stack, where a branch can have multiple child branches:

• Add support for tree structure in config file #252
  • Adds command to migrate from v1 to v2 format #263
    • Support tree structure in status command #262
      • Support tree structure when creating new branch #265
        • Support tree structure when adding branch to stack #270
      • Support tree structure when updating stack #264
      • Support tree structure when creating pull requests #266
      • Support tree structure when removing branch from stack #271
      • Support tree structure when cleaning stack #272

Fixes #256