Allow custom sorting when displaying several articles

A new form variable is accepted, "sort", to allow custom sorting when
several articles are rendered (default view, atom feed or list view).

The sort variable uses a very simple sort specification format:

[+-]?<field>?

Where "-" means reverse order, while "+" is regular, ascending, order.
Field specifies what article field will be used when sorting; title,
author, created, updated and uuid are accepted.

Both values are optional, and when one is omitted, a default will be used
according to the type of view being rendered.

Note: since "+" is used to encode " " in URLs, " " is accepted as an alias
for "+" to make easier to write URLs with sort specification manually.