Import/Export CSV

Might need to add the ability to import CSV files, depending on whether they've already used Excel to make their inventory.
Style preference.  If I'm looking for something that's a member of another object's collection, I usually add the query after the collection method.  e.g.

```ruby
location.items_in_stock.find_by(item_id: item)
```
Does the same thing, I just like how it reads a little better.  Might just be me.

By the way, you can use the object in the query instead of its id if you want: e.g.

```ruby
foo = Foo.find(1)
Bar.where(foo_id: foo)
# same as:
Bar.where(foo_id: foo.id)
```

Just make sure you know `foo` isn't nil, which could change what the query means (looking for bars with `foo_id = NULL`).  But in this case, we know `item` is not nil.