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

[dmalyuta]

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.