Use maktaba#path#Join()

This is the first maktaba commit!
Be slightly less xenophobic
Add Installation section & mention maktaba
Factor plugin-guard logic into vtd#Compatible()
Maktabize the commands and mappings

This will let users choose whether to enable or disable each, individually.
Add 'contexts' flag

It's a list of strings.  Contexts in the list will default to "included";
contexts preceded by a minus sign will default to "excluded".
Add Glaive instructions to README