Allow matching any line number of example in filter

[icy-arctic-fox]

The file and line filter should be able to match any line of the example block, not just the first. For instance:

it "does a thing" do # Line 1
  expect(true).to be_true # Line 2
end # Line 3

Running any of the following the match and run the example.

crystal spec spec/example_spec.cr:1
crystal spec spec/example_spec.cr:2
crystal spec spec/example_spec.cr:3

Implementing this involves adding a "end line" value to the Source type in addition to updating the example filter to match the line range.

Spawned from #18