Optional custom new tab URL

This resolves #979.
Automatically load content scripts into the options and blank pages

Load all appropriate content scripts by traversing the list in the
manifest. This removes the need to list them manually.
Merge branch 'customNewTabPage' of github.com:mrmr1993/vimium into mrmr1993-customNewTabPage
Cosmetic tweaks for @smblott-github
Merge branch 'customNewTabPage' of github.com:mrmr1993/vimium into mrmr1993-customNewTabPage
Touch up options page (newTabUrl).
Merge branch 'master' into customNewTabPage