remove webpack option

[jescalan]

So the webpack option is not necessary at all. postcss-loader handles setting the file root, which the include plugin pulls from, so it doesn't need to get the root passed in by the user. Removing this option entirely should mean that everything works exactly the same.

This PR will also include the appendPlugin and prependPlugin convenience methods.

[coveralls]

Coverage remained the same at 100.0% when pulling df8a8d8 on updates into 8adbf5e on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 4e71840 on updates into 8adbf5e on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 7b8d2d5 on updates into 8adbf5e on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 65fc3c0 on updates into 8adbf5e on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 60a3310 on updates into 21cdc6c on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 7d8d6df on updates into 21cdc6c on master.