Feature/runtime config

[zeyus]

Re-do of #234 but from a dedicated branch and targeting cboulay/apple_framework

On non-desktop platforms, often apps are sandboxed and don't have (read/write) access at all to $HOME, /etc/ or even the current working directory. These paths may not exist.

In addition, environment variables don't necessarily make a lot of sense in the context of a mobile app, and the application data paths might contain GUIDs or other aspects of the path that make it difficult to determine at compile-time.

This PR adds the ability to set a configuration file path for liblsl provided that the static member function lsl_set_config_filename is called before any other LSL function.

In addition, there's also an option to read config directly from a string stream via set_api_config_content which is loaded into the INI and discarded.