Unusable reporter plugins are indistinguishable from usable ones

New submission from jml <[email protected]>:

Some trial reporter plugins, such as the subunit plugin, depend on external software in order to work. Since this external software might not be installed on any given system that has trial, the plugins are unusable.

However, when you run a command like `trial --help-reporters`, the unusable plugins are indistinguishable from the usable ones.

Ideally, the unusable plugins would have some sort of marker indicating that they can't be used, and also provide some hints as to how they can be used (e.g. install subunit).

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Type     : enhancement
Component: trial
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/4164