Toggle pause with space + step a frame at a time with period

Based on initial work by by Kelvin Ly (@cactorium, https://github.com/cactorium) in https://github.com/asciinema/asciinema/pull/231/files

Closes #133
Closes #242

README.md

@@ -190,6 +190,12 @@ __Replay recorded asciicast in a terminal.__
 This command replays given asciicast (as recorded by `rec` command) directly in
 your terminal.
 
+Following keyboard shortcuts are available:
+
+- <kbd>Space</kbd> - toggle pause,
+- <kbd>.</kbd> - step through a recording a frame at a time (when paused),
+- <kbd>Ctrl+C</kbd> - exit.
+
 Playing from a local file:
 
     asciinema play /path/to/asciicast.cast

asciinema/player.py

@@ -26,14 +26,50 @@ def _play(self, asciicast, idle_time_limit, speed, stdin):
         stdout = frames.adjust_speed(stdout, speed)
 
         base_time = time.time()
+        ctrl_c = False
+        paused = False
+        pause_time = None
 
         for t, text in stdout:
             delay = t - (time.time() - base_time)
 
-            if delay > 0:
-                data = read_blocking(stdin.fileno(), delay)
-                if 0x03 in data:  # ctrl-c
-                    break
+            while stdin and not ctrl_c and delay > 0:
+                if paused:
+                    while True:
+                        data = read_blocking(stdin.fileno(), 1000)
+
+                        if 0x03 in data:  # ctrl-c
+                            ctrl_c = True
+                            break
+
+                        if 0x20 in data:  # space
+                            paused = False
+                            base_time = base_time + (time.time() - pause_time)
+                            break
+
+                        if 0x2e in data:  # period (dot)
+                            delay = 0
+                            pause_time = time.time()
+                            base_time = pause_time - t
+                            break
+                else:
+                    data = read_blocking(stdin.fileno(), delay)
+
+                    if not data:
+                        break
+
+                    if 0x03 in data:  # ctrl-c
+                        ctrl_c = True
+                        break
+
+                    if 0x20 in data:  # space
+                        paused = True
+                        pause_time = time.time()
+                        slept = t - (pause_time - base_time)
+                        delay = delay - slept
+
+            if ctrl_c:
+                break
 
             sys.stdout.write(text)
             sys.stdout.flush()