Add ValueReader for ConfigValue

Hi,

I just started using ficus and I thought it would be nifty to have a `ValueReader` for `com.typesafe.config.ConfigValue` so here it is. The underlying motivation is to read heterogeneous lists such as `[ "abc", { a = "def", b = 12 } ]` where `"abc"` is conventionally understood to be equivalent to e.g. `{ a = "abc", b = 1 }`