Implement `fetch_global_namespaces`

[halfak]

Fetches a JSON file containing information about all namespace 
names and aliases across all wikis using action=sitematrix and 
action=query&meta=siteinfo.  This file is used later by 
`aggregate` to parse page names into (namespace, title) 
pairs. 

Usage:
    fetch_global_namespaces (-h|--help)
    fetch_global_namespaces <api-host>

Options:
    -h --help   Prints this documentation
    <api-host>  URL for the MediaWiki host to query for 
                action=sitematrix