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/primitives/keyring/src/sr25519.rs

@@ -113,6 +113,33 @@ impl From<Keyring> for sp_runtime::MultiSigner {
 	}
 }
 
+#[derive(Debug)]
+pub struct ParseKeyringError;
+
+impl std::fmt::Display for ParseKeyringError {
+	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+		write!(f, "ParseKeyringError")
+	}
+}
+
+impl std::str::FromStr for Keyring {
+	type Err = ParseKeyringError;
+
+	fn from_str(s: &str) -> Result<Self, <Self as std::str::FromStr>::Err> {
+		match s {
+			"alice" => Ok(Keyring::Alice),
+			"bob" => Ok(Keyring::Bob),
+			"charlie" => Ok(Keyring::Charlie),
+			"dave" => Ok(Keyring::Dave),
+			"eve" => Ok(Keyring::Eve),
+			"ferdie" => Ok(Keyring::Ferdie),
+			"one" => Ok(Keyring::One),
+			"two" => Ok(Keyring::Two),
+			_ => Err(ParseKeyringError)
+		}
+	}
+}
+
 lazy_static! {
 	static ref PRIVATE_KEYS: HashMap<Keyring, Pair> = {
 		Keyring::iter().map(|i| (i, i.pair())).collect()