WIP: add sml_error function pointer, allow user to intercept error messages

[r00t-]

a basic attempt at fixing #17 .

replaces hardcoded fprintf() calls with an sml_error function pointer that can be overwritten by code using the library. a default is provided that mimics the old behaviour (write to stderr with a "libsml:" prefix and appended newline.

includes a test, test also demonstrates usage.

note that this breaks backwards compatibility (code assuming sml_error exists won't build or run with older versions of the library), so we should probably bump the version when merging this.

also we should consider if we maybe want a more sophisticated interface.

[r00t-]

@jahir: you beat me to mentioning this in #109 . it's sad that this is stalled for 1.5 years, but it got very little feedback after i submitted it. maybe you can add some? i'm mostly wondering if the API is sensible, see https://github.com/volkszaehler/libsml/pull/93#discussion_r549201545 .