module presence and order

This pull requests adds a new config key in `~/.lifebarrc` named `modules` where you can activate modules which will be rendered.

Example `~/.lifebarrc`:
```
modules            "bat0 fsone date time"
```
The items will be rendered in the order they have been specified in ~/.lifebarrc.

You can specify the following modules:
- time
- date
- ifone
- iftwo
- extip
- fsone
- fstwo
- bat0, bat1, ...
- therm0, therm1, ...