Add rudimentary regexp-based run by line number

[mattbrictson]

The Rails test runner has a robust run-by-line-number implementation that actually parses and understands the Ruby AST for test files.

For mighty_test, let's start with a simpler approach that hopefully covers the common use-cases. Given a line number, we look for method definition like def test_* or an Active Support style declaration like test "..." do. This is done with regular expressions; no fancy Ruby parsing.

There will certainly be edge cases where mighty_test will find the wrong test, or not find any test at all. But I think the feature works well-enough to ship as-is.