Add customImports command shorthand back

[tinder-matthewmourlam]

Mockolo command line argument for customImports changed where -c shorthand was removed.

Previously, you could use either --custom-imports OR -c: https://github.com/uber/mockolo/blob/d5f08f5e03bfc25f88adbe5bcab684ad2f25b45c/Sources/Mockolo/Executor.swift#L113-L114

But now only --custom-imports: https://github.com/uber/mockolo/blob/master/Sources/Mockolo/Executor.swift#L41-L43

This occurred on https://github.com/uber/mockolo/pull/153 but there is no explicit mention of this change so hoping we can add it back so folks do not need to update their existing cli scripts.

[CLAassistant]

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