var aArr = document.getElementsByTagName('a');
var host = document.location.host;

for(var i=0; i<aArr.length; i++)
{
    var aObj = aArr[i];
    var isLocalUrl = ( (String(aObj.href).indexOf(host) > -1) || (String(aObj.href).toLowerCase().indexOf('javascript') > -1) )
    
    if ( !isLocalUrl )
    {
        aObj.target = '_blank'
    }
}