Add context menu item for Yomitan

[jamesmaa]

I find some new users try to use Yomitan by selecting a word and right clicking it, hoping for a context menu item to pop up. I think this should be a valid entrypoint for looking up words or sentences. I suggest a context menu item that allows the user to open the pop up there OR open the search page popup.

Some documentation
https://developer.chrome.com/docs/extensions/reference/api/contextMenus

[jamesmaa]

Implemented in #1028