Maintains compatibility with mdbook 0.4.x. Includes comments for what to remove once 0.5.x is the only we care about. Some other changes technically could be changed at that point, but currently serve to enable universal support of 0.4.x and 0.5.x Fixes #1051. Change-Id: Ic5b405038d180bcd357bbd9e5716879e0c26e5f5