Support Subcategories in Kanji Categories

### Summary
**Issue** #17 

Now a kanji can have a `subCategory` property which is a string. 

When adding kanji to the selection box, it will check if the subCategory already exists in the DOM, if not, it will add it and move on to adding the kanji to that category. 

If it the subcategory is already in the DOM it will just move along and add that kanji to the specified subcategory. 

Categories can have the same subCategory and kanji will populate the correct subCategory.

In order to get around the fact the WaniKani's API does **not** return kanji in order via level, I wrote a function that takes the user's level and adds that many subCategory elements to the WaniKani category. Then it passes the kanji off to the Kanji selection updater, which sees these subCategories have already been added, and knows to not update the DOM. 

### TODO:
Note that this does **not** include selecting multiple kanji at a time. That should be in a separate pull request. Later I will have a separate .json file that contains Categories. Each Category will have an array of kanji, or a subcategory with an array.

:mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong::mahjong:
I'm sure you aren't able to see it in your tests, since your WaniKani level is still 1, but this for-loop is only creating the highest-level category (explanation below).  To see it yourself, you can temporarily change the wanikani URL to:

      url: 'https://www.wanikani.com/api/user/' + apiKey +  '/kanji/1,2,3,4,5',

$subCategory points to a single DOM object, and the loop is just repeatedly changing that same object's text, and the append() keeps moving it to the end, rather than creating a new object, because append() removes an object from its current location if it is already in the DOM.  The easiest fix is to clone() the original object:

  $category.append($subLevelTemplate.clone());