Document that you can use tab with `gi`

Closes #730
Import relevancy improvement code.
Address philc's comments.
Remove verbose commentary.
Fix options link in help dialog

This makes the options link in the help dialog cancel the default event
action, so the link (`href="#"`) isn't followed. The former behaviour
was most obvious when using the `LinkHints.activateModeToOpenInNewTab`
command, amoung others.
Merge pull request #1060 from smblott-github/relevancy

Relevancy - incorporate the position at which matches occur.
Merge pull request #1061 from mrmr1993/fixOptionsLink

Fix options link in help dialog
Remove unnecessary if statement
Import of bookmark-folder-search code and tests.
Strip top-level chrome folders.
Require bookmark folder separator to be first character of a query term.
Just ignore known and enumerated top-level folders.
Document top-level folders.
get new element position for each scroll tested element
Merge pull request #1063 from fent/master

get new element position for each scroll tested element
Merge pull request #1062 from smblott-github/bookmark-folders

Bookmark search by folder
Adding in search engines feature to fix #1009
Merge pull request #1058 from mijoharas/custom_search_engines

Custom search engines
Improve the help for saved searches
Reload content scripts after install/update

This loads the content scripts for pages when the extension is
installed or updated (or reloaded in developer mode). This means that
the new version of Vimium will become active immediately after loading,
and so tabs won't have to be refreshed.

As a devloper, this is amazing. Refreshing each tab individually after
each code change was a pain.