use browser built-in options_ui

[shinenelson]

Opening the extension's options_ui in a separate tab is an old method. Browsers now provide a built-in options_ui within the Extension's listing page itself.

This changeset changes the options_ui behaviour to open within the extension's listing context itself.

This can also avoid confusions like issue #35. I myself had the same confusion on Firefox. On Chrom{e,ium}, at least, there is a default button on the toolbar which the user can click and find the 'Options' page. On Firefox, there is none. Finding the 'Preferences' option within the overflow menu of the extension is too far-fetched to find. I got to #35 searching for how to change the theme on Firefox.

Resolves https://github.com/williambelle/github-contribution-color-graph/issues/35