Added manual POI option

Here's how to use it:
have a list called "manualpois" in your config filled with objects that contain
the attributes id, x, y and z. Feel free to add more attributes as you need them.
Then, write filter functions to filter for your custom POIs (hint: use the id).

Example config:
```python
worlds['world'] = "../Minecraft-Overviewer-Addons/exmaple"

def signFilter(poi):
    if poi['id'] == 'Sign':
        return "\n".join([poi['Text1'], poi['Text2'], poi['Text3'], poi['Text4']])

def townFilter(poi):
    if poi['id'] == 'Town':
        return poi['name'] + "\n" 

manualpois = [{'id':'Town', 'x':-259, 'y':64, 'z':150, 'name':'Ponyville'}]

outputdir = "../manualmarkertest"

rendermode = "smooth_lighting"

renders["render1"] = {
    'world': 'world',
    'title': 'exmaple',
    'markers': [dict(name="All signs", filterFunction=signFilter), dict(name="Towns", filterFunction=townFilter)]
}
```