convert filter definitions to taskKeys

This will allow to load the filter definitions from something else than
source code without having to reload the project after having changed the
filters.

sbtplugin/src/main/scala/com/typesafe/tools/mima/plugin/Keys.scala

@@ -14,9 +14,9 @@ class BaseMimaKeys {
   final val mimaFindBinaryIssues   = taskKey[Map[ModuleID, (List[core.Problem], List[core.Problem])]]("All backward and forward binary incompatibilities between a given module and current project.")
   final val mimaReportBinaryIssues = taskKey[Unit]("Logs all binary incompatibilities to the sbt console/logs.")
 
-  final val mimaBinaryIssueFilters   = settingKey[Seq[core.ProblemFilter]]("Filters to apply to binary issues found. Applies both to backward and forward binary compatibility checking.")
-  final val mimaBackwardIssueFilters = settingKey[Map[String, Seq[core.ProblemFilter]]]("Filters to apply to binary issues found grouped by version of a module checked against. These filters only apply to backward compatibility checking.")
-  final val mimaForwardIssueFilters  = settingKey[Map[String, Seq[core.ProblemFilter]]]("Filters to apply to binary issues found grouped by version of a module checked against. These filters only apply to forward compatibility checking.")
+  final val mimaBinaryIssueFilters   = taskKey[Seq[core.ProblemFilter]]("Filters to apply to binary issues found. Applies both to backward and forward binary compatibility checking.")
+  final val mimaBackwardIssueFilters = taskKey[Map[String, Seq[core.ProblemFilter]]]("Filters to apply to binary issues found grouped by version of a module checked against. These filters only apply to backward compatibility checking.")
+  final val mimaForwardIssueFilters  = taskKey[Map[String, Seq[core.ProblemFilter]]]("Filters to apply to binary issues found grouped by version of a module checked against. These filters only apply to forward compatibility checking.")
 
   final val mimaCheckDirection = settingKey[String]("Compatibility checking direction; default is \"backward\", but can also be \"forward\" or \"both\".")