Add LinkColumn

[philippemilink]

Hi,
The LinkColumn allows to display links in datatable cells.

Requires PRs #837.

[stephanvierkant]

@philippemilink I really don't understand why I didn't see this before, but there is some inconsistency between the LinkColumn and the interface it implements:
\Sg\DatatablesBundle\Datatable\Column\ColumnInterface::renderToMany(array &$row)
\Sg\DatatablesBundle\Datatable\Column\LinkColumn::renderToMany(array &$row, array &$resultRow):

Compile Error: Declaration of Sg\DatatablesBundle\Datatable\Column\LinkColumn::renderSingleField(array &$row, array &$resultRow) must be compatible with Sg\DatatablesBundle\Datatable\Column\ColumnInterface::renderSingleField(array &$row)

What are your thoughts?