allow a user to remove their token, via context menu and later options page

[dong-qian]

Should reset the access token request like first time install the extension.

[brumm]

@qiandongyq I'm sorry you were having issues!

For now you can switch to the extension's context in the console and call window.clearToken() to get back to the initial state. Here are some screenshots:

I'll add an entry to the extensions pop up menu to make this a bit more convenient until I add a full options page :)