TUI Difficulty stats for past few blocks (#805)

* added mining subview, changed main menu selection

* collecting difficulty stats from miner

* add diff calc view, separate server stats

* rustfmt

* block difficulty data output

* rustfmt

* ensure diff data is always shown

* don't write to stderr when tui running

util/src/types.rs

@@ -46,6 +46,8 @@ pub struct LoggingConfig {
 	pub log_file_path: String,
 	/// Whether to append to log or replace
 	pub log_file_append: bool,
+	/// Whether the tui is running (optional)
+	pub tui_running: Option<bool>,
 }
 
 impl Default for LoggingConfig {
@@ -57,6 +59,7 @@ impl Default for LoggingConfig {
 			file_log_level: LogLevel::Trace,
 			log_file_path: String::from("grin.log"),
 			log_file_append: false,
+			tui_running: None,
 		}
 	}
 }