Fix bug preventing configurable off-by-default.
Add functions to jump between diff hunks.
Extract method to avoid repetition.
Add commands for convenience.
Add shameless plug.
Improve portfolio link.
Check for existence of signs before clearing.

This fixes a bug whereby opening Vim on a non-git tracked file, then
calling `:DisableGitGutter` or `:ToggleGitGutter`, tried to clear a
variable before it had been defined.
Expose hunks.

Make hunks available to any other plugins which might want to use them.