Add support for NaNs and custom min/mix values
Add support for NaNs and custom min/mix values

Data series may now contain NaNs. NaN values will print a space
character instead of a spark char to indicate missing data:

    $ python sparkline.py 1 2 Nan 4 5
    ▁▂ ▆█

Users can now specify the min and max values that data points will be
clamped to:

    $ python sparkline.py 1 2 Nan 4 5 --min 2 --max 4
    ▁▁ ██

Fix argparse use to allow users to specify command line flags either
before or after the data series:

    $ python sparkline.py 1 2 Nan 4 5 --min 2 --max 4
    $ python sparkline.py --min 2 --max 4 1 2 Nan 4 5

RedKrieg/pysparklines