add helper function for width-aware slicing

requires patched wcwidth, see e87b1a495672053e8cbc0eb4f174560365308950
also prepares FmtStr to use width-aware, and preps for requiring unicode input
WIP using width-aware slicing for fmtstrs
tests passing
width as separate property
toolchain for colored string demos in place
Doc writing: FmtStr page written mostly
more docs, little new functionality
More docs!
bump version to 0.2.0

about to make the breaking change of requiring unicode
Merge branch 'new-docs'