Merge bitcoindevkit/bdk-cli#203: Add saving wallet config with `bdk-cli wallet config`

ceb93324b529207f2d46ad886df47af777d01f7f doc(wallet-init): update README.md examples (Steve Myers)
be31c14391176a9d38584f81082efbe540c0f70b feat(wallet-init): impl TryFrom for WalletOpts (Vihiga Tyonum)
7bf0982559f8810d96a93e848ef7ad8d4b6d871e feat(wallet-init): add `wallets` command (Vihiga Tyonum)
f3ee4ee4f72510b9cb9c7d3552f91c691e605a36 feat(init-wallet): rename init & add walletopts (Vihiga Tyonum)
f21729b3ecb1dd8bdc85527975fb4ed7347701a6 feat: add initializing wallet config (Vihiga Tyonum)

Pull request description:

  ### Description

  <!-- Describe the purpose of this PR, what's being adding and/or fixed -->

  This PR adds `bdk-cli wallet config` command to save wallet configuration information to `config.toml` file in the data directory.

  Fixes #192

  ### Notes to the reviewers

  - Reusing the exported `serde` crate from `bdk_wallet` did not offer the `derive` feature
  - Fix out-of-bound error for `shorten` fn

  ## Changelog notice
  - Add wallet subcommand `config` to save wallet configs
  - Add top-level `wallets` command  to show all saved  wallet configs
  <!-- Notice the release manager should include in the release tag message changelog -->

  ### Checklists

  #### Features

  * [x] command to save a wallet config, give wallet name or a default name is used, clap enforces required options; optional -f to override existing config: `wallet [-f] [-w <name>] config <wallet opts>`
  * [x] All other wallet commands require that a config exists, use wallet name or the default is used. Give error if config is missing: `wallet [-w <name>] sync | balance | new_address |` etc...
  * [x] Repl uses the same wallet configs: `repl [-w <name>]`
  * [x] command to list all saved wallet configs: `wallets`
  * [x] throw warnings if using mainnet and saving a private descriptor in a config.

  #### 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:

  * [ ] I've added tests for the new feature
  * [x] I've added docs for the new feature
  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  notmandatory:
    ACK ceb93324b529207f2d46ad886df47af777d01f7f

Tree-SHA512: f77ce3925d6cbd66a5fb24dbc7fdf1c40bb4f99b47825d165be4963bfde5eb876dcbd4266bf5b4e2fabdd66e3c7c0b42b09d899463558c18336969036f8f7f86