Only update if changes to documentation

This program is super useful! I have built it in to my git `pre-commit` hook so that it runs before every commit, but that means every single time I have to wait for the search index to "update" even when I haven't made any changes to the documentation.

It would be really cool to build in a functionality that the full program would only run if the documentation had changed. For instance, you could generate the new HTML, see if anything had changed, then only run the bulkier processes like creating the search index/building graphs if the HTML had changed (assuming that a change in the search index/graphs would also mean there was a change in the HTML).

There could also be cases where the HTML changed, but the search index/graphs did not. This would be a more complicated test (I assume), but would be the ultimate goal since updating the search index/graphs takes so long comparatively.

In the meantime, there is a workaround that I can just have my `pre-commit` hook call `ford` with different project files depending on whether or not I want the search index to be updated, but this is not ideal.