Add more tuning snippets

[rooby]

I guess this could end up being an excessively long list but handling some extra tunings would be useful. For example:

• Down 1, 1.5, 2 steps.
• Drop tunings down lower. Possibly this isn't as necessary since you can easily add a standard tuning for the appropriate steps down then manually change the one string. If there was a risk of too many snippets then I think these are less important.
• Other common tunings. It's debatable as to what is common enough I guess but I would say open tunings are fairly common.