Use std::bind instead of custom callback wrapper
Factor option parsing into separate function
Add include guards
Move include from header to implementation
Add mode of operation field to environment options
Variable scheduler queue size in non-realtime mode
Indentation fix
Add abstract worker interface
Remove unused include
Allow to pass a worker implementation to environment
Add virtual destructor for WorkerInterface
Remove superfluous CPP macros
Move sound file API registration to plugin API

Closes #23
Move sound file open method to Methcla_Host

This allows to iterate over all available sound file APIs until a file
can be opened.
Add missing source files for dummy sound file API
Add write method to sound file API
Add plugin sources stub for methcla pro
Remove methcla pro build definitions
Fix include guard
Return nullptr from plugin library function