fix summation and substraction from options

This patch allows to increment and decrement variables within sections, inherited sections and conditional sections.
For example, you have a section, that creates a json like that:
`{"key-1": "value-1", "key-2": "value-2", "key-3": "value-3"}`
```ini
[make-json]
recipe = recipes:jsonize
json = key-1 value-1 key-2 value-2 key-3 value-3
```
and you want to add a part `{"key-4": "value-4"}` in Mac OS X:
```ini
[make-json:macosx]
json += key-4 value-4
```
It will not increment! It will create a separate invalid variable `json +` into **.installed.cfg**
And you will not be able to **buildout** again, because **buildout** will say: "Sorry, guy, `json +` is invalid format"