Add basic logging facility

[mr-smidge]

Add a means to allow the library to log information that can be combined with the logging functions of an application using the library.

Prefer something simple without external dependencies, similar to the idea in this SO post: https://stackoverflow.com/questions/1136323/adding-generic-logging-support-in-c-shared-library

Points to note:

• Avoid external dependencies if possible.
• Allow a console logger as an easily-selectable default (?) choice.
• Support info()/warn()/error() methods.
• Maybe support logging levels, avoid string formatting if not going to be printed.