Plugin loader could offer the option to load all known plugins

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-plugin/issues/4)) by Louise Poubel (Bitbucket: [chapulina](https://bitbucket.org/%7B5cfa2075-477b-4ded-bdb9-8d2479544ec4%7D/), GitHub: [chapulina](https://github.com/chapulina)).**

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Migrated from [ignition common issue 15](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-common/issues/15) (ignitionrobotics/ign-common#15).

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The plugin loader added on [ign-common pull request 27](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-common/pull-requests/27/adds-pluginloader) offers, among other things, interfaces to:

* add paths where to look for plugins
* load a library from a previously added path, given its filename
* list all loaded libraries

If one wants to list all the libraries available on the path, one needs to load all of them first. It would be nice if the plugin loader offered a function which automatically loads all libraries found in all registered paths.