As a user, I want to filter my shopping list to make it easier to locate an item in the list

Description

We added an input field to the top of List.js. As a user types in the field, the list is filtered to show only the items that match the filter.
We also added a button with an 'X' next to the filter. When the X is clicked, the filter is reset to an empty field.

closes #9

[x] Display a text field above the top of the shopping list
[x] As the user types into the field, the list should narrow to display only items that contain the text the user entered in the filter field
[x] When the field has text in it, the user should be able to tap a UI element (e.g., with an "X" button next to the field) to clear the field
[x] The filter text should match any part of the item name (i.e. it should not only match from the start of the string)