Add ExampleFilter class

src/spectator/example_filter.cr

@@ -0,0 +1,24 @@
+module Spectator
+  # Checks examples to determine which should be run.
+  class ExampleFilter
+    # The criteria for filtering examples can be multiple types.
+    # A `String` will exact-match an example's name.
+    # A `Regex` will perform a regular expression match on the example's name.
+    # A `Source` will check if the example is in the specified file on a given line.
+    # An `Int32` will check if an example is on a given line.
+    alias Type = String | Regex | Source | Int32
+
+    # Creates the example filter.
+    # The *criteria* should be a list of what to filter on.
+    def initialize(@criteria = [] of Type)
+    end
+
+    # Checks if an example is in the filter, and should be run.
+    def includes?(example)
+      return true if @criteria.empty?
+      @criteria.any? do |criterion|
+        example === criterion
+      end
+    end
+  end
+end

src/spectator/includes.cr

@@ -28,6 +28,7 @@ require "./config"
 require "./config_builder"
 require "./config_source"
 require "./command_line_arguments_config_source"
+require "./example_filter"
 
 require "./example_failed"
 require "./expectation_failed"