Bolt-expressions

Fix
* Import all subpackages in __init__ for conveniency ([`7c2ed2c`](https://github.com/rx-modules/bolt-expressions/commit/7c2ed2c65d80b1e0f91efd970724414c24fd3e91))

Fix
* Generate negative score constants in init function ([`9ec47cb`](https://github.com/rx-modules/bolt-expressions/commit/9ec47cb3b7aa4e1d0baf865a2fa29a902eae1790))

Feature
* Implicitly convert sources into text components ([`d27870e`](https://github.com/rx-modules/bolt-expressions/commit/d27870ef068b27dddf054b01d6c418d608a0a61a))
* Add `component` method to convert sources into text components ([`a22caf9`](https://github.com/rx-modules/bolt-expressions/commit/a22caf9256c234944ff1de41829d48e7c37ddc9b))

Feature
* Require bolt_expressions.contrib.commands by default ([`f748ee3`](https://github.com/rx-modules/bolt-expressions/commit/f748ee3ea2e6cc5eb9416d295ce39001d03856da))
* Add bolt_expressions.contrib.commands ([`e40ae95`](https://github.com/rx-modules/bolt-expressions/commit/e40ae9562bae5589a0c541a60884ce79211febf8))

Feature
* Let custom objectives be added to init function ([`99483fb`](https://github.com/rx-modules/bolt-expressions/commit/99483fbbcd4c3653b40cc3eed0c948843699caea))

Fix
* Generate init function only when needed ([`01f3193`](https://github.com/rx-modules/bolt-expressions/commit/01f3193d56d79c62608ff239b8f26032c1cc2685))
* Prevent init function from having unused objectives ([`4b6a490`](https://github.com/rx-modules/bolt-expressions/commit/4b6a490b47345e5ee66e386c839aebc052c22ac4))
* Prevent init function from having duplicated/unused constants ([`05a2638`](https://github.com/rx-modules/bolt-expressions/commit/05a2638a8e9f637f443b2a98419544b3abd75127))