Extended position reporting with both home and work coordinates. Home position now retained after reset. Other minor changes/fixes.

- Grbl now tracks both home and work (G92) coordinate systems and does
live updates when G92 is called.
- Rudimentary home and work position status reporting. Works but still
under major construction.
- Updated the main streaming script. Has a disabled periodic timer for
querying status reports, disabled only because the Python timer doesn't
consistently restart after the script exits. Add here only for user
testing.
- Fixed a bug to prevent an endless serial_write loop during status
reports.
- Refactored the planner variables to make it more clear what they are
and make it easier for clear them.

stepper.c

@@ -139,7 +139,7 @@ static uint8_t iterate_trapezoid_cycle_counter()
 // interrupt doing its thing, not that big of a deal, but the latter cause is unknown and worrisome. Need
 // to track down what is causing this problem. Functionally, this shouldn't cause any noticeable issues
 // as long as stepper drivers have a pulse minimum of 1usec or so (Pololu and any Allegro IC are ok).
-// This seems to be an inherent issue that dates all the way back to Simen's v0.6b.
+// ** This seems to be an inherent issue that dates all the way back to Simen's v0.6b or earlier. **
 
 ISR(TIMER1_COMPA_vect,ISR_NOBLOCK)
 {