CLI API refactoring and improvement (#4692)

It changes the way we extended the CLI functionalities of substrate to allow more flexibility. (If this was not clear, here is another version: it changes the `sc_cli` API to allow more flexibility).

This touches a few important things:
 - the startup of the async task with tokei:
    This was in node and node-template and I moved it to substrate. The idea is to have 1 time the code that handles unix signals (SIGTERM and SIGINT) properly. It is however possible to make this more generic to wait for a future instead and provide only a helper for the basic handling of SIGTERM and SIGINT.
 - increased the version of structopt and tokei
 - no more use of structopt internal's API
 - less use of generics

Related to #4643 and https://github.com/paritytech/cumulus/pull/42: the implementation of "into_configuration" and "get_config" are similar but with better flexibility so it is now possible in cumulus to have the command-line arguments only of the run command for polkadot if we want

Related to https://github.com/paritytech/cumulus/issues/24 and https://github.com/paritytech/cumulus/issues/34 : it will now be possible to make a configuration struct for polkadot with some overrides of the default parameters much more easily.

substrate/bin/node/cli/src/lib.rs

@@ -39,11 +39,15 @@ mod browser;
 mod cli;
 #[cfg(feature = "cli")]
 mod factory_impl;
+#[cfg(feature = "cli")]
+mod command;
 
 #[cfg(feature = "browser")]
 pub use browser::*;
 #[cfg(feature = "cli")]
 pub use cli::*;
+#[cfg(feature = "cli")]
+pub use command::*;
 
 /// The chain specification option.
 #[derive(Clone, Debug, PartialEq)]