Lectern

Feature
* Add define directive ([`85ec3cc`](https://github.com/mcbeet/lectern/commit/85ec3ccdd682f35c0e3556e116dab48cb55a27a9))
* Move require and script directives into their own beet plugins ([`a1e33cd`](https://github.com/mcbeet/lectern/commit/a1e33cd1312040794cb11900b0c64ae1835208b5))

Fix
* Pack.png url now uses a static revision ([`1f7013c`](https://github.com/mcbeet/lectern/commit/1f7013cad5b6bd185322da5fb5880f285df30de5))

Feature
* Clean error message for invalid urls ([`7964d24`](https://github.com/mcbeet/lectern/commit/7964d24ba0991cfa8a8b8d40d426a295075117ae))
* Add script directive ([`81603f9`](https://github.com/mcbeet/lectern/commit/81603f9b845e178d011e66c70f85495dcaaf00ea))
* Add directive escaping ([`5fbd90c`](https://github.com/mcbeet/lectern/commit/5fbd90c8d95a341a56942cd6d4df1e73f4da31b9))
* Add download modifier ([`77f0864`](https://github.com/mcbeet/lectern/commit/77f086407a9393a735bec32fc437e4b45a097893))

Fix
* Serialize namespace extra ([`94f1995`](https://github.com/mcbeet/lectern/commit/94f1995808d9be25c5a604132b99aff7554984ba))

Feature
* Add sound and particle directives ([`87aa5f9`](https://github.com/mcbeet/lectern/commit/87aa5f981bac2db1289ffd55d583e6f5af818108))

Fix
* The snapshot testing example was not using run_beet properly ([`3c933e8`](https://github.com/mcbeet/lectern/commit/3c933e81080a1180b31d2940afca023ac9a7daf3))

Feature
* Add prepend modifier ([`5e1bb1e`](https://github.com/mcbeet/lectern/commit/5e1bb1e81835b8a8aaaedab0f1d2fb138bd3d993))