Custom tone colors

[mreichhoff]

Now that there's a way of setting more-durable preferences (see #7), and other preferences are in progress (see #10), it likely makes sense to allow users to customize the colors they use for each tone. This way, those used to, e.g., Pleco's tone colors can use them easily.

Basic approach: set up an input[type=color] for each tone, save the values in storage.sync, read the values in the content script and side panel. For an example implementation from another project, see the equivalent in HanziGraph. A reset button might also be good, but is optional.