feat: configurable auto attach

[martinra]

In my own workflow I prefer to attach gitsigns manually whenever I need it. This adds a configuration flag to accommodate this.

[lewis6991]

Can I ask why you prefer to attach manually? What is the downside of always attaching?

[martinra]

Of cause. For me there are two downsides to always attaching. First, since I mostly use neogit and gitsigns only in very specific situations, I like to keep the editor slim. Second, when writing I like to keep the side/annotation column for diagnostics, but also don't want to give up the annotations that gitsigns provides when I use it.