Don't sanitize official plugin names, only g:installed_{plugin}

Plugin names will no longer be sanitized by converting all special characters to underscores. This allows the exact names to be listed in Glaive tab completion, fixes some inconsistencies with the name accepted by `maktaba#plugin#Require`, and cuts down on the potential for plugins with similar names to clash. Only the automatically defined variable `g:installed_{plugin}` will have the plugin name sanitized anymore.

BTW, this doesn't break backwards compatibility any more than fixing `Require` to expect the un-sanitized name would. Some plugin names will change, but that shouldn't impact much behavior.

Resolves #55.