extract_sources works with Windows paths

Prior to this, backtrace lines were simply split by a single colon.

Unfortunately that is also the drive letter delimiter in Windows paths which resulted in a lot of empty source fragments of "C:0". ("C" from the drive letter and 0 from `"/path/to/rails/file.rb:16".to_i`)

Now the trace line is split by the first colon followed by some digits, which works for both Windows and Unix path styles.
You are right, that would be a good precaution.

BTW: How do you produce a bare `"(irb)"` as a trace line? All I could provoke was `"(irb):1"` (i.e. with a colon and a number, which is matched by the pattern)