Allow one build info object per configuration. (new feature)

[asflierl]

It's been useful in the past to have seperate build info objects for different configurations (like - keep test API keys for test and things like that). So far, I've always duplicated the plug-in code in my projects when I had to do that so I'd like to propose this change to make this feature possible without much overhead.

Example usage:

lazy val root = (project in file(".")).enablePlugins(BuildInfoPlugin)

name := "meep"
version := "1.0"
scalaVersion := "2.11.6"

addBuildInfoToConfig(Test)

buildInfoObject := "Meep"
buildInfoPackage in Test := "meep"
buildInfoKeys in Compile := Seq(name, version)
buildInfoKeys in Test  := Seq(scalaVersion)

The buildInfoObject setting here works for both configs, since it's the default fallback. The others were adapted per-config. Hope it's clear what I mean. :)