Add support for options

[Pycea]

Provides a way to add options to the extension. Options are objects that implement the options API, which requires a key and a default value, with the following functions being optional:

• onValueChange(newValue): called whenever the option value is changed
• onStart(currentValue): called when the page is first loaded
• onLoad(currentValue): called after the page has loaded
• processHeader(commentData, headerElem): called on each comment header
• processComment(commentData, commentElem): called on each comment

Saving and loading option values is handled automatically.

No options are implemented in this branch, but see here or other branches to see what they might look like.

Other changes

• Adds a popup for setting options, though it's currently empty
• Changes the extension to run at document_start, so that options can load CSS without flickering
• Adds the storage permission (this won't trigger a permissions warning)

[maksverver]

Sorry about the delay. This change looks great overall (thank you for preparing it!), with two notes:

1. There are a few small things I'd like to refactor, like avoiding global variables where possible, and normally I'd ask for that on the pull request, but since you have some follow-up changes prepared, I think it's better not to break those and refactor afterward.

2. The changes in ext-comments.js break demo.html, which I use a lot for local testing.

I'll go ahead and merge this as is, and feel free to send me more pull requests for some of the actual option implementations you have in mind. Meanwhile, I'll try to fix demo.html which shouldn't be too difficult.