Add favicons to vomnibar completions

Favicons are displayed as a background-image style next to urls in
similar style to chrome's history page.
For tabs the icon is fetched from the url specified by chrome tab's
favIconUrl property. Unfortunately extensions to not have access to
chrome://favicons/ to get bookmark favicons but fortunately most sites
serve a favicon from protocol://hostname/favicon.ico
(e.g. http://example.com/favicon.ico) so this url is tried for non-tab
types.
Add tests to validate command structures.
Merge pull request #1197 from smblott-github/command-option-tests

Add tests to validate command structures.
Reduce max removeTab to chrome.session.MAX_SESSION_RESULTS.
Merge favicons for vomnibar.
Fix tests for favicons.
Validate each advanced command is in a command group.
Ensure advanced commands are in command groups.
Merge pull request #1199 from smblott-github/command-option-tests

Command tests - validate advanced commands
Note need for more command tests.
Merge branch 'command-option-tests'
Repair insert mode.

I broke it here: 77e1ded091062ca2e52264d222482dcd06290a9b
Revert 2c7bebb5f2c873850c2b2d82013cab4eb3d4913c

On reflection, this seems too dangerous:

   - Somebody excluded flash for a reason, and without knowing that
     reason, it seems dangerous.

   - Imagine a flash game with key bindings.  Perhaps it uses ? to
     show the key bindings, and ESC to hide them again.  With
     2c7bebb5f2c873850c2b2d82013cab4eb3d4913c, you can never hide the
     key bindings (I think).

All in all, this just seems too risky.
Fix tests
Merge branch 'fix-tests' of github.com:mrmr1993/vimium
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).