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.
Cosmetic tweaks for @smblott-github
Touch up options page (newTabUrl).
Don't skip installing listeners.

Fix listener installation bug.  To see bug, do:

   - open vimium options page
   - use 'b' to select and go to a bookmark
   - use <alt-LEFT_ARROW> to return to options page
   - vimium key bindings are now *not* installed

Bug was introduced in 28e9c8374bfc7a4fd479dbb958a4a87331fb0857.
Fix refresh open tabs and state.

Fixes refresh of state bug introduced in
046650ba22ac12ebb18d1eb109fe694016eb77c0.
Help text for non-empty text options.
Remove "Restore to defaults" button.

And line up main frame with the first two columns of the main options
table.
Move "Custom key mappings" above the fold.
Activate "Save" button on input.
Activate "Save" button on input (non-empty text areas).
Tweaks to options page.
Move custom search engines above the fold.
Reduce the size of the excluded URLs container.
Delete rule button for exclusion rules not tab selectable.
Reposition save button such that "always" on screen.
Control-ENTER in text option blurs and saves all options.
Options help text and grammar.
Do not line wrap some text options.
Tone down help messages in footer.