Add #path method

Update #to_s to use relative path if possible.

src/spectator/source.cr

@@ -11,10 +11,30 @@ module Spectator
     def initialize(@file, @line)
     end
 
+    # The relative path to the file from the current directory.
+    # If the file isn't in the current directory or a sub-directory,
+    # then the absolute path is provided.
+    def path
+      # Add the path separator here.
+      # Otherwise, things like:
+      # `spectator/foo.cr` and `spectator.cr` overlap.
+      # It also makes the substring easier.
+      cwd = Dir.current + File::SEPARATOR
+      if file.starts_with?(cwd)
+        # Relative to the current directory.
+        # Trim the current directory path from the beginning.
+        file[cwd.size..-1]
+      else
+        # Not trivial to find the file.
+        # Return the absolute path.
+        file
+      end
+    end
+
     # String representation of the source.
     # This is formatted as `FILE:LINE`.
     def to_s(io)
-      io << file
+      io << path
       io << ':'
       io << line
     end