Fix crash after home bug

Fixes a bug in the Extruder Runout Prevention feature that caused the
extruder to move back to “current_position” after a move if it was
activated while the move was in progress. For long home moves (which
are longer than the dimensions of the machine’s working area), this
would cause the machine to crash into the far end of its travel after
homing. This usually occurred on the Z axis, which could result in
damage to the machine if you don’t hit the reset button in time.
Remove previous_millis_cmd update from extruder runout prevention code

This seems to defeat the purpose of previous_millis_cmd, preventing the
time elapsed from previous_millis_cmd from ever reaching
max_inactive_time or stepper_inactive_time while the heat was on.
Previous commit borked.
Merge pull request #725 from whosawhatsis/Marlin_v1

Fix bugs in Extruder Runout Prevention, including DAMAGING HEAD CRASH
Format README.md with subsections

Having actual section headers instead of just bold text makes those parts of the document individually linkable.
Merge pull request #726 from DanLipsitt/patch-1

Format README.md with subsections
Merge branch 'Marlin_v1' into mendel90