af_comics: split filters to separate files to ease maintenance

plugins/af_comics/af_comics_template.php

plugins/af_comics/filter_base.php

plugins/af_comics/filters/af_comics_cad.php

plugins/af_comics/filters/af_comics_comicpress.php

plugins/af_comics/filters/af_comics_dilbert.php

plugins/af_comics/filters/af_comics_explosm.php

plugins/af_comics/filters/af_comics_gocomics.php

plugins/af_comics/filters/af_comics_pa.php

plugins/af_comics/filters/af_comics_twp.php

plugins/af_comics/init.php