Alias default url

[marcolgl]

Hi,
this PR addresses the need for a default url to replace the one
specified when creating an alias, when no input is given to the
omnibar other than the alias.
This feature, which i miss from vimiumc, is to set an url for
those site for which you want an alias, but also redirect to
another url (e.g. an homepage) when you don't provide a
result to search for.
I implemented that as an user setting, to avoid changes to
the already crowded signature of addSearchAlias :

addAliasDefaultUrl(alias, default_url)

where alias must be a defined alias.
Thanks for your consideration and for your work

surfingkeys-demo.mp4