Feature custom sidecar 1123 (#1129)

* Working on custom sidecar, #1123

* Working on custom sidecar, #1123

* Added custom sidecar example

* Added WRITE_SKIPPED argument for --sidecar-template

* Updated report writer for user sidecars

* Added noqa to long lines in report writer

* Initial tests for --sidecar-template

* Initial tests for --sidecar-template

* Initial tests for --sidecar-template

* Completed tests for --sidecar-template

* Added json example for --sidecar-template

* Added json example for --sidecar-template

tests/custom_sidecar.mako

@@ -0,0 +1,22 @@
+<%doc>
+    This is an example Mako template for use with --sidecar-template
+    For more information on Mako templates, see https://docs.makotemplates.org/en/latest/
+
+    The template will be passed three variables for rendering:
+    photo: a PhotoInfo object for the photo being exported
+    photo_path: a pathlib.Path object for the photo file being exported
+    sidecar_path: a pathlib.Path object for the sidecar file being written
+</%doc>
+
+<%def name="rating(photo)" filter="trim">\
+    % if photo.favorite:
+        ★★★★★
+    % else:
+        ★☆☆☆☆
+    % endif
+</%def>\
+
+Sidecar: ${sidecar_path.name}
+    Photo: ${photo_path.name}
+    UUID: ${photo.uuid}
+    Rating: ${rating(photo)}