Implement Snackbar Component

[traviskaufman]

Spec Reference: https://www.google.com/design/spec/components/snackbars-toasts.html

It's been called a "Toast" in the past, but the thing that's not Android-specific is a snackbar.

• Initial Implementation
• Dark Theme (background: #fafafa, text: text-primary-on-dark, action: theme primary color)
• Keyboard Focus Handling
• In demo, ensure that "Action on Bottom" is unchecked and disabled when "multiline" is unchecked
• End-to-end usage example in README (instantiation, using it in response to an event)
• Start-aligned snackbars (left for LTR, right for RTL)
• Add dismissOnAction option, which defaults to true. When true, snackbar dismisses as soon as a user interacts with its action button (see Implement Snackbar Component #28).
• Address Action label misaligned in RTL Snackbar #564

Copied from google/material-design-lite#4490

[abraham]

It doesn't look like left-aligned for tablet/desktop is supported or on the todo list. Is that planned?

[Garbee]

Left-aligned is new to the spec. A new issue should be made to track adding it as a modifier.

[traviskaufman]

THanks @abraham and @Garbee. We actually think it makes the most sense to roll this issue into the TODO list here. Closing #382 and putting as an item.