This adds several improvements the previously introduced
`services.weechat` module:
* Dropped `services.weechat.init` as the initialization script can now
be done on package-level since 2af41719bc using the `configure`
function.
* Added `sessionName` option to explicitly configure a name for the
`screen` session (by default: weechat-screen).
* Added `binary` option to configure the binary name (e.g.
`weechat-headless`).
* Added docs regarding `screen` session and `weechat.service`.
