Add options to not loop back to first hunk, go to first/last hunk

I frequently find that I'd like to: 1. Go to the first or last diff hunk 2. Scroll from one hunk to the next _without looping back to the first hunk once I am visiting the last hunk_ I imagine that these options would be easy to day. I can imagine a new customization variable like `git-gutter:loop-to-start` for bullet 2, and functions `git-gutter:go-to-first-hunk` and `git-gutter:go-to-last-hunk` for bullet 1.