Merge pull request #183 from MikePopoloski/master

Updating README and fixing device issue
Added D3D_FEATURE_LEVEL_11_1 define when it's not defined.
D3D11: Moved missing defines into header.
Cleanup.
Changed imguiTabs() api. Now you can set additional 'enabled' flag per tab.
Merge pull request #184 from dariomanesku/master

Changed imguiTabs(). Now you can set additional 'enabled' flag per tab.