Expose version-choosing logic in use_release_issue()

[jennybc]

Supports calling use_release_issue() without knowing the right version in advance, i.e. helps you specify the version:

> proj_set("~/rrr/readxl")
✔ Setting active project to '/Users/jenny/rrr/readxl'
> use_release_issue()
Current version is 1.2.0.9000.
Which part to increment? (0 to exit) 

1: major --> 2.0.0
2: minor --> 1.3.0
3: patch --> 1.2.1
4:   dev --> 1.2.0.9001

Selection: 2
✔ Opening URL 'https://github.com/tidyverse/readxl/issues/553'

[hadley]

👨‍🍳 😗