Twerp Should Allow External Runners

[bigeasy]

Currently, the twerp program has a --runner parameter that is defined using OptionParser as an enum. The enum is populated by a directory listing of lib/runner. This assertion means that, in order to provide a customized test runner, an end user must place the customized runner in the Twerp library.

It would probably be better to allow the user to specify a runner external to Twerp.

Perhaps the --runner parameter can accept a library path. The module would return a CustomRunner class or a createRunner method.