XS_PLUGIN and XS_FILTER implementation

This patch introduces following features:
- XS_PLUGIN context option to add plugins to libxs
- XS_FILTER option to switch between different filter types
- Automatic loading of plug-ins is *not* implemented.

From the implementation point of view:
- standard prefix filter is implemented as a pluggable filter
- trie_t and mtrie_t are joined into a single class
- the code for 0MQ/3.1 compatibility is left in in the form of comments
- new test for testing re-subscriptions is added
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

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