Add horizontal support for lazy-url

Lazy-url can now optionally function with horizontal scrolling.

To get this functionality set the 'lazy-url-horizontal' data attribute
to 'true'.

You can also optionally set a new context by setting the
'lazy-url-context' data attribute. This defaults to the window, so it's
useful to override if the scrolling container doesn't take up full width
or height.

These two options are useful for creating a horizontal scrolling gallery
for example.

Change-Id: Ifc13b8de6fd6a2c9b5afd3fb9fd4d7d7bf26f085
Reviewed-on: https://gerrit.causes.com/26461
Reviewed-by: Nebojsa Petrovic <nebs@causes.com>
Tested-by: Nebojsa Petrovic <nebs@causes.com>