Allow specifying preprocessor order

[ISSOtm]

Fixes #1172

[ISSOtm]

I've determined that the order in which preprocessors are run is non-deterministic (blame topological-sort), and that makes one of the tests randomly fail since it expects an exact order.

The question then is: is this acceptable?

[ehuss]

Thanks! I think it would be ideal if the order would be at least stable. Would it be possible to sort the preprocessors by name?

[ISSOtm]

Sure, I'll see about stabilizing ties.

[ISSOtm]

Should it be made a contract in the documentation, or should we specify that ties are guaranteed to be identically resolved across runs but not across versions?

[ISSOtm]

Additionally, do we match, or ignore case?

[ehuss]

I think mentioning that they are otherwise sorted by name would be fine.
I wouldn't worry about ignoring case, just doing a normal string comparison would be fine.

[ISSOtm]

So sticking with the default string sorting should be enough, right?

[ehuss]

Yea, the default string sort should be good.

[ISSOtm]

Okay, I updated the test to expect index before links, and the documentation using your almost exact wording. Ready for review!

[ISSOtm]

Done!

[ehuss]

Thanks!