Add possibility to automatically prioritize Methods

[fohrloop]

Motivation:

• The current development version of wakepy 0.8.0 has very simple logic for method prioritization: Priority is the highest if the Method is the WAKEPY_FAKE_SUCCESS method, then is 1 (low) if the method is not supported on the current platform and 0 (higher) if the method is supported. Then the methods are sorted alphabetically (case insensitively).
• Would be nice to be able to let the Methods to poke around (read env variables, check for available binary, etc.) and to guess their priority to be higher or lower.

Tasks:

• [ ] Implement some method on Method class, for example Method.get_priority_modifier(), which should return an integer.
• [ ] Document the prioritization usage.

This is not urgent now as the number of available methods is so low, but could be useful in the future.