Missing plural rules

[ErdaradunGaztea]

Plural rule parsing requires a rule definition and it may not always be present (for one reason or another). First of all, give a better error message than this:

Error in if (!grepl("^[^~,]+~[^~,]+(,[^~,]+~[^~,]+)*$", definition)) { : 
  argument is of length zero

The other notes include:

• maybe implement default plural rules as per this page and store the transformed contents as .Rds?
• maybe require plural rules lazily, not forcing the evaluation until the rule is necessary?
• maybe export some functions related to plurality so that the user has more control over it?