As of rustc 1.26, the `flatten()` method on iterators is preserved by
the rust standard library.
This could cause this code to hard-error some time in the future with
the `flatten()` function actually implemented by the standard library.
Hence we move to use the `Itertools::flatten()` function here
explicitely.
Before the configuration object (the raw TOML object) was provided via a
wrapper object `Configuration`. This was ugly and not very nice to use.
Now, we only have the `toml::Value` object we lend out from
`Runtime::config()`.
The changes included libimagrt internal rewrites, which are not visible
to the user. Anyways, this change changes the API for config-fetching
from the runtime, so fixes for all other crates may follow.
The changes also removed the support for reading the "editor" setting
from the configuration file, which was not used anyways (in the example
imagrc.toml file).
The CLI-reading and ENV-reading are still supported, though.
