Use standard context-menu notation and capitalization

[cprcrack]

Hi there, some years ago I was about to develop and publish an extension myself with this very same functionality. However I found yours and that's what I've been using.

However since the first day I was quite unhappy with the inconsistency of the context-menu notation and capitalization between this extension and the rest of the software I use.

It took me some time to make this step but here's the PR that fixes it in case you can accept it and publish a new version to the Chrome Web Store. Otherwise no worries :)