Add ImageMagick Clock plugin (#46)

* Add ImageMagick Clock plugin

ImageMagick Clock draws a custom clock using ImageMagick (or any other CLI image drawing tool).

* Address review comments

* Update thumbnail

* Add description to image_width/height settings

* Guard the mv command to fix race condition

* Handle time jump (resume from sleep)

imagemagick-clock/translations/en.json

@@ -0,0 +1,16 @@
+{
+  "settings": {
+    "cmdline": {
+      "label": "Command line",
+      "description": "The command should generate an image file to be displayed as the clock. `%output_file` will be replaced with the path to the output file. Then `%H`, `%M`, etc. will be replaced with the current time values. See `man strftime` for more formatting options."
+    },
+    "image_width": {
+      "label": "Image width",
+      "description": "The display width of the clock image in logical pixels. (Not the width of the generated image file.)"
+    },
+    "image_height": {
+      "label": "Image height",
+      "description": "The display height of the clock image in logical pixels. (Not the height of the generated image file.)"
+    }
+  }
+}