Variable sortable Paramater

[inkitt]

This PR allows the sortable parameter to be a variable. It uses the same 'concept' for the variable title parameter to make sortable also variable.

This is needed for cases when you have a very flexible table whose column values are changeable.

// Code
$scope.variableColumn = 'name';

// HTML
    <table ng-table="tableParams" class="table">
        <tr ng-repeat="user in users">
            <td data-title="variableColumn">
                {{user[variableColumn]}}
            </td>
            <td sortable="'age'">
                {{user.age}}
            </td>
        </tr>
    </table>

[coveralls]

Coverage remained the same when pulling 2fbe898 on AliAlbazaz:variable-sortable-param into fc7716a on esvit:master.

[coveralls]

Coverage remained the same when pulling 4f0e50f on AliAlbazaz:variable-sortable-param into fc7716a on esvit:master.

[coveralls]

Coverage remained the same when pulling d266206 on AliAlbazaz:variable-sortable-param into fc7716a on esvit:master.

[coveralls]

Coverage remained the same when pulling d266206 on AliAlbazaz:variable-sortable-param into fc7716a on esvit:master.