add user define configs

ffplayout/config/README.md

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+# Custom Configuration
+
+Extend your arguments for using them in your custom extensions.
+
+The file name must have the **argparse_** prefix. The content should look like:
+
+```YAML
+short: -v
+long: --volume
+help: set audio volume
+```
+
+At least **short** or **long** have to exist, all other parameters are optional. You can also extend the config, with keys which are exist in **ArgumentParser.add_argument()**.
+
+**Every argument must have its own yaml file!**

ffplayout/config/argparse_volume.yml

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+short: -v
+long: --volume
+help: set audio volume