Streaming pager: initial commit. Fixes pallets#409

The streaming pager works using generators.

Just pass a generator function or object into echo_via_pager:

generator expression:
```
click.echo_via_pager(("{}\n".format(i) for i in range(999999)))
```

generator function / expression:
```
def gen():
    counter = 0
    while True:
        counter++
        yield counter
click.echo_via_pager(gen)
click.echo_via_pager(gen()) # you can pass both
```

click/_termui_impl.py

@@ -13,6 +13,7 @@
 import sys
 import time
 import math
+import inspect
 from ._compat import _default_text_stdout, range_type, PY2, isatty, \
      open_stream, strip_ansi, term_len, get_best_encoding, WIN, int_types, \
      CYGWIN
@@ -300,6 +301,25 @@ def pager(text, color=None):
         os.unlink(filename)
 
 
+def _pipe_make_gen(text):
+    """Converts string or generator of strings into generator"""
+    if inspect.isgeneratorfunction(text):
+        for chunk in text(): yield chunk
+    elif inspect.isgenerator(text):
+        for chunk in text: yield chunk
+    else:
+        yield text
+
+
+def _pipe_make_str(text):
+    """Converts string or generator of strings into string"""
+    if inspect.isgeneratorfunction(text):
+        return "".join(text())
+    if inspect.isgenerator(text):
+        return "".join(text)
+    return text
+
+
 def _pipepager(text, cmd, color):
     """Page through text by feeding it to another program.  Invoking a
     pager through this might support colors.
@@ -318,17 +338,19 @@ def _pipepager(text, cmd, color):
         elif 'r' in less_flags or 'R' in less_flags:
             color = True
 
-    if not color:
-        text = strip_ansi(text)
-
     c = subprocess.Popen(cmd, shell=True, stdin=subprocess.PIPE,
                          env=env)
     encoding = get_best_encoding(c.stdin)
     try:
-        c.stdin.write(text.encode(encoding, 'replace'))
-        c.stdin.close()
+        for chunk in _pipe_make_gen(text):
+            if not color:
+                chunk = strip_ansi(chunk)
+
+            c.stdin.write(chunk.encode(encoding, 'replace'))
     except (IOError, KeyboardInterrupt):
         pass
+    else:
+        c.stdin.close()
 
     # Less doesn't respect ^C, but catches it for its own UI purposes (aborting
     # search or other commands inside less).
@@ -351,6 +373,7 @@ def _tempfilepager(text, cmd, color):
     """Page through text by invoking a program on a temporary file."""
     import tempfile
     filename = tempfile.mktemp()
+    text = _pipe_make_str(text)
     if not color:
         text = strip_ansi(text)
     encoding = get_best_encoding(sys.stdout)
@@ -364,6 +387,7 @@ def _tempfilepager(text, cmd, color):
 
 def _nullpager(stream, text, color):
     """Simply print unformatted text.  This is the ultimate fallback."""
+    text = _pipe_make_str(text)
     if not color:
         text = strip_ansi(text)
     stream.write(text)

click/termui.py

@@ -1,6 +1,7 @@
 import os
 import sys
 import struct
+import inspect
 
 from ._compat import raw_input, text_type, string_types, \
      isatty, strip_ansi, get_winterm_size, DEFAULT_COLUMNS, WIN
@@ -215,10 +216,12 @@ def echo_via_pager(text, color=None):
                   default is autodetection.
     """
     color = resolve_color_default(color)
-    if not isinstance(text, string_types):
+    if not inspect.isgenerator(text) \
+      and not inspect.isgeneratorfunction(text) \
+      and not isinstance(text, string_types):
         text = text_type(text)
     from ._termui_impl import pager
-    return pager(text + '\n', color)
+    return pager(text, color)
 
 
 def progressbar(iterable=None, length=None, label=None, show_eta=True,