Allows customizing Jekyll _config.yml file

[juanpedromoreno]

In three different ways (not exclusive each other):

• Specifying a provided _config.yml
• Specifying a YAML string inline in the sbt configuration
• Through custom liquid variables

These three ways will be merged in order to generate the final and single _config.yml file.
This is possible thanks to the ConfigYml case class and the sbt setting named as micrositeConfigYaml, which looks like follows:

case class ConfigYml(
      yamlCustomProperties: Map[String, Any] = Map.empty,
      yamlPath: Option[File] = None,
      yamlInline: String = ""
  )

For instance:

micrositeConfigYaml := ConfigYml(
  yamlCustomProperties = Map("org" -> "My Org"),
  yamlInline = """exclude: [package.json, grunt.js, Gruntfile.js, node_modules]
|""".stripMargin,
  yamlPath = Some((resourceDirectory in Compile).value / "microsite" / "myconfig.yml")
)

This PR fixes #72 and #43.

Please, @raulraja @jdesiloniz could you review? Thanks!

[jdesiloniz]

LGTM!!