Add new --simplify-proportion option.

This allows the proportion of retained points to be specified for
simplification, which may be friendlier than specifying an exact area.

This also includes a fix for NaN areas that were occurring due to small
negative values in the triangle area calculation (being passed to sqrt).
Error checking for --simplify-proportion.
Allow comma-separated --properties. Fixes #30.
Use defaults when missing from options.

Fixes #16 #17.
Merge branch 'simplify-proportion'