Enhancment of isloggedin/isnotloggedin output filter

Allow to specify a context for the isloggedin/isnotloggedin output filters, the default is still the current context, so it shouldn't break old implementations or implementations where no context is specified.

with that change it is possible to do something like that:

```
[[!+modx.user.id:isloggedin=`mgr`:eq=`1`:then=`class="loggedin"`:else=``]]
```

when before just the current context was checked (as far as I understand)