Mechanism to query current config

The `:Glaive` command makes it handy to update flag values, but we currently don't have a user-level interface for checking current flag values (calling functions like `:echo maktaba#plugin#Get('someplugin').Flag('someflag')` doesn't count as a user-level interface).

Many vim commands like `:set`, `:let`, `:command`, `:map`, etc. have a no-arg version that spits out the current config. Glaive should adopt a convention like this to let people query/explore their plugin config.

This could entail a `:Glaive` command to list all flags and their values by plugin, a `:Glaive SOMEPLUGIN` command to list flags for an individual plugin, and `:Glaive SOMEPLUGIN SOMEFLAG` to show the current value of an individual flag.