Merge bitcoindevkit/bdk-cli#74: Bump version to 0.4.0 and add tag v0.4.0 for release

8b417a302a5dc7fb4bc64a143a138d81ae1d67de Bump version to 0.4.0 (Steve Myers)
dab5d825f1a4496a38222545a4dd3744a9487dc0 Update Cargo.lock (Steve Myers)

Pull request description:

  ### Description

  Bump version to 0.4.0 and add tag v0.4.0 for release.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk-cli/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  rajarshimaitra:
    Review + tACK https://github.com/bitcoindevkit/bdk-cli/pull/74/commits/8b417a302a5dc7fb4bc64a143a138d81ae1d67de

Tree-SHA512: aeb60b2800b6c46aac325747aec053249d01ab80e78d48ef0f579251f9d82dc191ce4bb0f9ff2a968acd151d8482bb494a98c5c8bb889eb2d03f25394e36bcfd