* Added a function to add callbacks that are called when a logger is registered
* Fix non captured registration 2 not being properly tested for
* Replace std::list by std::vector
* Remove const refs to shared pointers
* Fix missing header
This method applies levels which is set by environment variable
`SPDLOG_LEVEL` to the a single controller. Usefull for loading
configuration into manually created loggers.
* Added: `spdlog::get_level()` API function - like `logger::level()`, except for the name change
* Added: `spdlog::should_log()` API function - like `logger: should_log()`
The meaning of using quotes to #include is implementation defined, so it
may or not may be what we want. At least POSIX
(https://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html)
says: "headers whose names are enclosed in double-quotes ( "" ) shall be
searched for first in the directory of the file with the #include line",
so not what we want since "spdlog" ends up twice in the path.