image_filter support crop_keepx/crop_keepy

crop_keepx reduces the image by width and crops extraneous edges by height.
crop_keepy reduces the image by height and crops extraneous edges by width.
Bugfixed: rewrite directive do not use ngx_http_named_location
Merge remote-tracking branch 'alibaba/master'
Merge pull request #455 from yzprofile/master

Bugfixed: rewrite directive do not use ngx_http_named_location
Merge pull request #448 from Lax/feature/image_filter_crop_keep

image_filter支持crop_keepx/crop_keepy