Byoc

Fix
* Don't share bound getters between instances ([`2d72a0e`](https://github.com/kalekundert/appcli/commit/2d72a0e6d8a2b198c8439b834ea4749ea517601b))

Documentation
* Organize README into sections ([`d674072`](https://github.com/kalekundert/appcli/commit/d6740725450c541b8102f104ee849d6be1feffce))
* Link to examples from stepwise_mol_bio ([`d45b44a`](https://github.com/kalekundert/appcli/commit/d45b44a8705036f5d2638d54c6d99afc9def2adf))

Fix
* Make layers mutable again ([`7b23c01`](https://github.com/kalekundert/appcli/commit/7b23c010b713182a258b8d98fc3475175ec19979))

Documentation
* Fix doctest ([`6e0c25b`](https://github.com/kalekundert/appcli/commit/6e0c25b8d6f187325461c346f8194a74ed3c9698))

Feature
* Use `Getter` classes to pick values ([`aa18b52`](https://github.com/kalekundert/appcli/commit/aa18b525339048603d0f7d75b93add1b020b1232))

Feature
* Add `SelfConfig` ([`39e3f3b`](https://github.com/kalekundert/appcli/commit/39e3f3b6126e93892f005d66121143adb570ede8))
* Make it easier to compose cast functions ([`2ef93e2`](https://github.com/kalekundert/appcli/commit/2ef93e2899921bfa25bea0ccf76fe40b88cefad4))
* Force layer values to be collections ([`dd82715`](https://github.com/kalekundert/appcli/commit/dd827158a7d69b28ca2bfbc95b493a1d79845d2d))

Feature
* Make key names optional ([`3a50d13`](https://github.com/kalekundert/appcli/commit/3a50d13c0c32966dfb22155f3caba58a30da992d))
* Allow subkeys to be specified using tuples ([`ac970dc`](https://github.com/kalekundert/appcli/commit/ac970dc6d6587f0b294596d8b84cb92f60540011))
* Teach FileConfig to read paths from attributes ([`328544c`](https://github.com/kalekundert/appcli/commit/328544cdfac14d19455dcc4cb3b0f11499adceb9))

Fix
* Respect default cast argument when using Key() ([`0442d69`](https://github.com/kalekundert/appcli/commit/0442d6954ae86a58a0e4371f28ab48f5db3c1207))