Added Grbl state in status report. Removed switch support.

- Added Grbl state (Idle, Running, Queued, Hold, etc) to the real-time
status reporting feature as feedback to the user of what Grbl is doing.
Updated the help message to reflect this change.

- Removed switches (dry run, block delete, single block mode). To keep
Grbl simple and not muddled up from things that can easily be taken
care of by an external interface, these were removed.

- Check g-code mode was retained, but the command was moved to '$C'
from '$S0'.

config.h

@@ -123,7 +123,7 @@
 // that do not and must not exist in the streamed g-code program. ASCII control characters may be 
 // used, if they are available per user setup. Also, extended ASCII codes (>127), which are never in 
 // g-code programs, maybe selected for interface programs.
-// TODO: Solidify these default characters. Temporary for now.
+// NOTE: If changed, manually update help message in report.c.
 #define CMD_STATUS_REPORT '?'
 #define CMD_FEED_HOLD '!'
 #define CMD_CYCLE_START '~'