Spec generation appends modified liquid patches after previous ones

[dylanahsmith]

When running either the generate:liquid_ruby or generate:standard_filters rake tasks, Helpers.insert_patch will be used to append a patch to liquid code under tmp/liquid if the patch isn't already included in that file. That means that if the file is modified, then it won't find it included and append the modified patch after the previous patch. This can lead to confusion and/or incorrect results when modifying that patch.

[dylanahsmith]

It would be cleaner to just do runtime monkey patching rather than actually modifying the source file. That would also allow us to use git submodules to manage these dependencies, rather than having this hidden persistent state to manage in a temporary directory.