Pass filters argument to compile funcs and PreProcessor

[moyogo]

Continues #466 and is needed for googlefonts/fontmake#737.

This also adds BaseFilter.pre to specify if the filter is a pre-filter or a post-filter. It is False, for post-filter, by default, as loadFilter() puts filters in postFilters by default.

[anthrotype]

Thanks, I left some comments. Also, we'd need tests for these new feature

[anthrotype]

maybe add another test in integration_test.py testing that the filters parameter is actually passed through to the preprocessor from all the compile* call sites

[moyogo]

maybe add another test in integration_test.py testing that the filters parameter is actually passed through to the preprocessor from all the compile* call sites

Good catch, some filters args were missing.