mraa: initial implementation of "hooks"

* Should allow for more platform quirks to be handled by mraa without
* massive conditional areas per platform.

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

docs/platform-hooks.md

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+Hooks can be defined per supported platform to allow for highly custom operations if needed.
+This feature of MRAA should only be used by developers defining the board definitions, NOT an end user.
+
+## Types of Hooks
+
+** REPLACE **
+Defining a replace function will entirely replace the associate function. This should only be done if your new function can handle everything the mraa function would normally.
+
+** PRE **
+Any functionality defined here will be performed when the main function is called.
+
+** POST **
+Any functionality perfomed here is done just before the normal function returns. All post functions will have passed into them the return value that would normally be returned.
+
+## Hooks
+** gpio **
+* init (pre-post) - On RAW functions
+* mode (replace-pre-post)
+* dir (replace-pre-post)
+* write (pre-post)
+* use-mmaped (replace-pre-post)