Enhance line number's UX

[conao3]

Re #113.

@conao3 Yes, feel free to close this PR, as I don't intend to work on it anymore.
If I may, I'd recommend rewriting the line numbers to use text-properties to store and retrieve them, and perhaps using overlays to optionally display them. I think that will improve performance and flexibility.

[cireu]

Not only make it optional, current line number implementation has awful User Experience.

1. Any regex that matching a digit will match line number. Mentioned in Can digits in line numbers be excluded in matching? #45

2. Misalignment, old implementation simply add one space after line number, so the swoop buffer will look like

I had made some changes in https://github.com/cireu/helm-swoop fix issues I mentioned above. But I still need some time to test and merge them.

[cireu]

and perhaps using overlays to optionally display them.

Can overlays be put on empty lines? @alphapapa