Added template_filter.py to examples

examples/template_filter.py

@@ -0,0 +1,17 @@
+""" Example of using a custom python function as an osxphotos template filter 
+
+    Use in formath:
+    "{template_field|template_filter.py::myfilter}"
+
+    Your filter function will receive a list of strings even if the template renders to a single value.  
+    You should expect a list and return a list and be able to handle multi-value templates like {keyword}
+    as well as single-value templates like {original_name}
+"""
+
+from typing import List
+
+def myfilter(values: List[str]) -> List[str]:
+    """ Custom filter to append "foo-" to template value """
+    values = ["foo-" + val for val in values]
+    return values
+