Gridlines and inverted mode

Two switches in the `plot()` function which allow to:

* add gridlines with `gridlines=true`
* invert the plot with `invert=true`
* both

It comes in handy if Braille blank, `0x2800`, has different width than
the remaining characters in the range.

Example without gridlines:
![image](https://cloud.githubusercontent.com/assets/3216223/3083674/0683b96a-e4e7-11e3-88bf-d26deb0b890b.png)

Example with gridlines:
![image](https://cloud.githubusercontent.com/assets/3216223/3083676/1683aac8-e4e7-11e3-8db3-17c0c962282f.png)