Adds syntax to make easier the use of command aliases

[juanpedromoreno]

This PR brings some syntax helpers to make easier the use of command aliases.

For example, given this list of command aliases:

val commandAliases: Seq[Def.Setting[_]] =
    addCommandAlias("validate", ";clean;validateJS;validateJVM") ++
      addCommandAlias("validateDocs", List("docs/tut", "readme/tut", "project root").asCmd) ++
      addCommandAlias("validateCoverage", ";coverage;validate;coverageReport;coverageOff") ++
      addCommandAlias("validateJVM", List(
        "m1JVM/compile",
        "m2JVM/compile",
        "m1JVM/test",
        "m2JVM/test",
        "project root").asCmd) ++
      addCommandAlias("validateJS", List(
        "m1JS/compile",
        "m2JS/compile",
        "m1JS/test",
        "m2JS/test",
        "project root").asCmd)

Basically, asCmd will convert the module/command into: ;project module;command.

Hence, you could define orgScriptTaskListSetting setting safely in this way:

orgScriptTaskListSetting := List(
        orgValidateFiles.asRunnableItem,
        "validateDocs".asRunnableItemFull,
        "validateCoverage".asRunnableItemFull
      )

[fedefernandez]

Looks good