Support '+' as prefix for integer

[jylitalo]

Right now, if I have define

        opts.integer '--modify-capacity',
                     'Modify the desired ECS cluster size (defaults to zero, which means no change)',
                     default: 0

and call it with command --modify-capacity +50, slop doesn't recognise it as 50, but instead uses 0, which is default value. Simple change into regular expression should solve this issue (unless it breaks some other functionality in slop).

[leejarvis]

Thanks @jylitalo -- please could you add a test for this change?

[leejarvis]

Thanks for your contribution @jylitalo!