Issue #913: Merge --master and --plugin switches

Merged master/plugin switches into content in openmw
and mwiniimporter.

Extension in content files is now required.

Signed-off-by: Lukasz Gromanowski <lgromanowski@gmail.com>
Issue #913: Merge --master and --plugin switches

Merged master/plugin switches in launcher.

Signed-off-by: Lukasz Gromanowski <lgromanowski@gmail.com>