Newtab.urlEX (Custom new tab & bookmark browser)

Now featuring a shiny new bookmark browser!

** This is the Web Extensions version of the old newtab.url **

Since Firefox removed the ability to have a custom new tab page (previously accomplished by setting "browser.newtab.url"), I decided to try and write an add-on to restore it.

Some of the APIs are still missing that I need to write it to the same functionality as the original, but as of now you can set the new tab url, and there's an option to attempt to make the page active (as opposed to the location bar).

Note regarding location bar highlight / clearing: Due to a change made in Firefox awhile back, the URL in the location bar isn't automatically highlighted or cleared when opening a custom new tab. They're working to fix this - the feature is being tracked at Bug 1372996 on Bugzilla. Currently, it's not expected to be shipped in time for Firefox 57, but I'll make sure to add it as soon as it's available!