highlighting does not work with github code

[7flash]

**To Reproduce
Steps to reproduce the behavior:

1. Go to 'https://github.com/raindropio/app/blob/master/src/index.js'
2. Click on 'Save to highlights' in context menu of selected line

Expected behavior
It's expected to appear in my highlights as normally with selecting other text on the page, but it does not work when selecting the text inside of code block.

[exentrich]

It's bit complicated because Github uses textarea. Unfortunately I don't have reliable solution yet to fix this use-case