add functions to manage indexes
Merge branch 'indexfuncts' of https://github.com/colophon/clarango into colophon-indexfuncts

Pull indexfunctions by colophon into new branch to review changes
add periods and formatting to index docstrings to match the docs of other namespaces
Merge branch 'colophon-indexfuncts'
update README: simplify feature list, add index functions