provide setting to avoid overriding '] and friends

```'[` and `'[` are useful for going back to the beginning of a large paste operation, or a `:r!` operation. Although I'm ok with opinionated, sane defaults, I also think plugins should strive to avoid overriding default mappings out-of-the-box.

So, I think the ideal solution might be to not map `'[` and friends by default. But this pull request doesn't take a stand on that. Rather, it just provides an additional option to disable these 4 mappings.

Although this is kind of ugly, it would be even uglier to require users to explicitly set `g:SignatureEnableDefaultMappings = 1` *and* then have to manually specify the other mappings (which *are* reasonable defaults).

I'd also suggest that cycling through marks in alphabetical order isn't such an important feature that it warrants overriding `'[`.