Increase flexibility of news title matching

[hadley]

Included a decent amount of refactoring, first to make the regular expression more readable, and then to make news_version return something that was easier to test.

Fixes #980

[jennybc]

This would be great to merge, so we can see NEWS info re: development version.

[jayhesselberth]

Looks like there's a test failure specifically on R 3.2:

── 1. Error: github links are added to news items (@test-build-news.R#8)  ──────
unused argument (perl = TRUE)
1: data_news(pkg) at testthat/test-build-news.R:8
2: news_version(titles)
3: regexec(pattern, x, perl = TRUE)

Alternatively we could use rematch2::re_match() here.

[jennybc]

Works great!