Burst boost, feel improvement & cool speedtech

If and when there will be graphics, visual cue will be needed.

include/conf.h

@@ -18,3 +18,4 @@
 #define JUMP_BUFFER 10
 #define JUMP_GRACE  10
 #define AIR_JUMPS   3
+#define BURST_BOOST 0.5

src/player/update.c

@@ -87,6 +87,8 @@ int player_update(struct Player *player)
 		else {
 			player->jumps_left -= 1;
 			player->spd_y = AIR_JMP_SPD;
+			/* burst boost */
+			player->spd_x += (float)dir_x * BURST_BOOST;
 		}
 		player->air_state = AirRising;
 		player->jump_buffer = 0;