Converters can run before liquid is evaluated [#889]

(A response to #889)

Specified converters can now be run before liquid tags are evaluated by including the method `run_before_liquid!`. Tests and docs have been updated to reflect this, and the `PreIdentity` converter has been added as a lowest-priority catch-all for pre-liquid converters. The new workflow is:

1. Run pre-liquid converters
2. Evaluate liquid tags
3. Run post-liquid (default) converters

Note that this means that posts may go through two different converters - one pre-liquid and one post-liquid.

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Converters are added to the post-liquid queue by default. This means that if you don't add any pre-liquid converters, behaviour should default to standard.

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Unit tests run fine on my machine, and manual tests seem to show that everything is working well. Feedback appreciated.