Phoenix.Naming.underscore/1

Parsed documentation:
Converts String to underscore case

## Examples

    iex> Phoenix.Naming.underscore("MyApp")
    "my_app"

    iex> Phoenix.Naming.underscore("my-app")
    "my_app"

In general, `underscore` can be thought of as the reverse of
`camelize`, however, in some cases formatting may be lost:

    Naming.underscore "SAPExample"  #=> "sap_example"
    Naming.camelize   "sap_example" #=> "SapExample"
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