Remove BOM
Fixes
* [plugin.h](include/clap/plugin.h): advice for plugin id and version strings
* [host.h](include/clap/host.h): clarify when it is allowed to call get_extension()
* [CMakeLists.txt](CMakeLists.txt): the target `clap-test` now includes `clap-plugin-template`
+* Remove UTF-8 BOM from a few files
# Changes in 1.1.4
