Usage: implement supported_filetypes (#53)

* Add supported_filetypes to Usage class.

* Usage example of supported_filetypes

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---------

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examples/Usage-cli_filetypes.yaml

@@ -0,0 +1,6 @@
+---
+method: cli
+scope: meta-only
+command: parse --type=example --summary {{ file_path }}
+supported_filetypes:
+    - example

schemas/base.yaml

@@ -161,14 +161,22 @@ classes:
                     - >-
                       For `method: python`, a parametrized function invocation which
                       should be available after installing and setting up the `Extractor`.
-                    - The templated parameters which can be requested from the user
+                    - >-
+                      The templated parameters which can be requested from the user
                       are [described by the `UsageTemplate` class](UsageTemplate.md).
             scope:
                 required: false
                 range: UsageScope
                 description: >-
                     Specification of extraction scope.
-
+            supported_filetypes:
+                required: false
+                multivalued: true
+                description: >-
+                    An enumeration of the `FileType` that this `Usage` of an `Extractor`
+                    supports, matching one or more of the `FileTypes` present in the
+                    `supported_filetypes` slot of the `Extractor`. Defaults to all
+                    `supported_filetypes` specified in the `Extractor` definition.
 
     Installation:
         description: >-