Permalinks compatibility

[criticalmash]

Issue #16 brought up the idea of using permalinks with Assemble-Navigation. Navigation was designed to guess the final paths of pages based on their literal position in the source directory. Permalinks break that pattern by changing the view's path. Another issue is that most permalink packages work as pipeline plugins, which make it difficult to predict at what stage of the process they are being called.

If a views path is changed by a permalink function after the onLoad middleware has done it's work, then navigation will have the wrong path to the page.