Disable on PDF pages and on chrome web store page

[jeromepl]

Users are expecting this to work on PDFs, but I haven't found a way to make that work yet. For now, disable the extension on open PDF pages.
Should be a simple change to the manifest.

[jeromepl]

Needs a bit more than a change to the manifest:

• content scripts can be disabled from the manifest using "exclude_matches": ["*://*/*.pdf"], but that assumes that every path that ends with .pdf is actually a pdf...
• context menus need to be disabled through the api chrome.contextMenus.create
• the popup icon can be grayed out through a browser action (?)