add add-feature

Hi,

I've added a small extension in which one can add a value to a comma separated value list.

e.g.
```
[Section]
DirsAllowed = /home/me, /etc/app
```
```
crudini --set --add my.ini Section DirsAllowed "/home/other"
```
will result into 
```
[Section]
DirsAllowed = /home/me, /etc/app, /home/other
```