Adminimize export / import role(s)

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This pull request fixes issue #.

Hi Frank, this PR add the possibility to export / import adminimize option as a json for one or more roles. Now we can export all roles ( as it used to work ) or some choosen roles to not reset all at the import when it's not necessary. Don't hesitate to let me know if you want me to do some changes.

wp-media / adminimize