Increate download chunk size of 256k (#657)

With 8k the download status if updated way faster than is
necessary.

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index 1ac1e05..f344830 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -694,7 +694,7 @@
       # Draw a progress bar 80 chars wide (in non-TTY mode)
       progress_max = 80 - 4
       progress_shown = 0
-      block_sz = 8192
+      block_sz = 256 * 1024
       if not TTY_OUTPUT:
           print(' [', end='')
       while True: