Fixes and improvements to b-form-textbox

• added optional formatter feature
• lazy formatting now happens on blur event, to get around issues with IME composition
• better auto rows/resizing based on CSS height rather than number of newlines
• adds in Boolean trim and number props to emulate v-model modifiers .trim and .number
• custom v-model event (update)
• $emits input event with arg of (optionally formatted) value
• $emits change event with arg of (optionally formatted) value
• $emits blur event with arg of native event
• attach all native listeners (except input, change, blur) directly on <textarea> element
• improved event $emit ordering
• move common code of <b-form-textarea> and <b-form-input> to mixin to minimize codebase
• change test suite to use vue-test-utils
• • basic structure, classes and attributes
• • events
• • formatting
• • trim and number modifier props
• • auto rows/height cannot test getComputedStyle in JSDOM test environment

Fixes #1708
Closes #1710
Fixes #1915
Fixes #1470

To-Do:

• have b-form-input use the new mixin, and to follow the new event model via separate PR