Dedicated i18n audit command #167

Looking at #39 again, I feel like it might be easier to have a dedicated command for the audit part, e.g. wp i18n audit.

Then make-pot doesn't have to deal with this, and we can have all the different output formats that we need. Also, it makes it easier to add further audits.

For example, one audit that I could think of is string similarity. If there are two strings which are quite similar, but only different in casing or punctuation, it could suggest merging the strings. Levenshtein distance alone might work for that.