Allow multiple process_record() calls per scan

This is particularly relevant for, e.g., the ergodox EZ and
other keyboards with slow scan rates. Without changing the API or
behavior of individual process_record() calls, we allow a
configuration flag to make multiple calls in a single scan.

This will probably have miniscule effects on non-steno users,
and it's not enabled by default for any keyboards. Added note
about it to ergodox README.

Signed-off-by: seebs <seebs@seebs.net>

keyboards/ergodox_ez/readme.md

@@ -31,3 +31,8 @@ To flash with ´teensy-loader-cli´:
 
   - Press the Reset button by inserting a paperclip gently into the reset hole
     in the top right corder.
+
+## Settings
+
+You may want to enable QMK_KEYS_PER_SCAN because the Ergodox has a relatively
+slow scan rate.