Add an option to suppress all online-fallbacks and fall gracefully instead

[phseiff]

Using an online formula converter (and potentially an online HTML-to-PDF converter in the future; see #31) as fallbacks if the required dependencies are not installed is one of gh-md-to-html's strengths, but can be tricky if one works with sensible files one doesn't want to upload on some internet service, especially since incorrectly setting up gh-md-to-html's dependencies (or, even worse, gh-md-to-html wrongly detecting these dependencies) could potentially lead to this happening on accident.

It might therefore be good a good idea to add an option --no-online-fallbacks to gh-md-to-html that turns off online fallbacks, causing gh-md-to-html to fail gracefully if a dependency is missing instead (whether this would imply and/or require -o OFFLINE or -o OFFLINE+ by default or just not care about the core converter at all is up for debate, though).