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

core/src/global.rs

@@ -29,6 +29,7 @@ use consensus::{BLOCK_TIME_SEC, CUT_THROUGH_HORIZON, DIFFICULTY_ADJUST_WINDOW, I
                 MEDIAN_TIME_WINDOW};
 use core::target::Difficulty;
 use consensus::TargetError;
+use util::LOGGER;
 
 /// Define these here, as they should be developer-set, not really tweakable
 /// by users