Configurable controls

[sjrct]

The controls that can be specified in the configuration file are incomplete. All controls should be configurable. Once done, the documentation in doc/config.md should reflect the changes. All that needs to be done is basically replicate the existing code in src/config.c to allow for the configuration of additional controls; it is really just busy work.

[BahaVv]

An update on this: controls are now configurable thanks to work from Macri-man, but due to some redundancies between his code and Richteer's, this issue will not be marked as resolved until it's possible to CLEANLY rebind and add new controls.

[sjrct]

The documentation in config.md still needs manual updating, and default values for the controls should be in a config file found outside the code, but now the only changes necessary to add a new control are adding an enum value in src/controls.h and an equivalent line in src/controls.c. Adding automatic documentation generation might make the former easier, but for now the issue is resolved as the controls are simplified to the point where adding one implies that it is configurable.