Adds config option (missing_value_fill_truncation). Fix #150

Adds config option (missing_value_fill_truncation) which controls which
values (x, y, or either) in a series are checked for missing values when
determining where to place the closing point of a filled series path.

Commissioned by ClearSpark (http://www.clearspark.co.za)
Merge pull request #151 from sirlark/fill_truncation

Adds config option (missing_value_fill_truncation). Fix #150