Added wx.feature function to use on configure.lua

After some thinking and looking into the configure --help output I wrote 3 functions: wx.feature, wx.enablefeature and wx.disablefeature.

wx.feature is to easily execute a given code if a feature is enabled or disabled, it does all checks of the flag automatically so we dont have to add repetitive code and checks.

Also I was thinking that we should add to the list of options in options.lua the platforms the option is applicable if not generic, and default = yes if the feature should be enabled by default.

On this commit you will see an example usage of wx.feature and some options altered to serve as an example for the above suggestions. Also commented the new functions to make the idea clearer.