ENH: Perform download of ITK tag file during build time

Use a cmake custom command to run a CMake script during build time to
download and unzip the download.

Change-Id: I31e4b122868bae736b3b425296712a4cecde035d
Merge topic 'DoxygenWithITKTagsDuringBuild' into next

22b9f5b4 ENH: Perform download of ITK tag file during build time
ENH: Update superbuild to ITKv4.6rc03

Change-Id: I6f4e0b4ba51cbbfc0a241b8d50c7d4d41fa24799
Merge topic 'SIMPLEITK-543_UpgradeITKTowards4.6' into next

c7425de6 ENH: Update superbuild to ITKv4.6rc03