Make sure 'drush omega-export' does not mess with the formatting of the .info file.

Currently, 'drush omega-export' still messes with the formatting of the .info file slightly. Previously (in 3.x) it was even worse as we messed with the entire .info file. Now, in 4.x we at least limited that to the settings[] part of the .info file. However, I think we can do even better and not mess with it at all. So... The plan is to replace existing settings[] entries in place through regex and only append stuff to the end of the .info file if there is no existing placeholder for it yet.