Fix pretty_format_json to use int indent

The indent parameter for json should be integer and under Python2 is
will raise an error if not. So switch from str to int and mention
default value in help text.

pre_commit_hooks/pretty_format_json.py

diff --git a/pre_commit_hooks/pretty_format_json.py b/pre_commit_hooks/pretty_format_json.py
index 9115077..fb1c487 100644
--- a/pre_commit_hooks/pretty_format_json.py
+++ b/pre_commit_hooks/pretty_format_json.py
@@ -33,24 +33,12 @@
         f.write(new_contents)
 
 
-def parse_indent(s):
-    # type: (str) -> str
+def parse_num_to_int(s):
+    """Convert string numbers to int, leaving strings as is."""
     try:
-        int_indentation_spec = int(s)
+        return int(s)
     except ValueError:
-        if not s.strip():
-            return s
-        else:
-            raise ValueError(
-                'Non-whitespace JSON indentation delimiter supplied. ',
-            )
-    else:
-        if int_indentation_spec >= 0:
-            return int_indentation_spec * ' '
-        else:
-            raise ValueError(
-                'Negative integer supplied to construct JSON indentation delimiter. ',
-            )
+        return s
 
 
 def parse_topkeys(s):
@@ -68,9 +56,12 @@
     )
     parser.add_argument(
         '--indent',
-        type=parse_indent,
-        default='  ',
-        help='String used as delimiter for one indentation level',
+        type=parse_num_to_int,
+        default='2',
+        help=(
+            'The number of indent spaces or a string to be used as delimiter'
+            ' for indentation level e.g. 4 or "\t" (Default: 2)'
+        ),
     )
     parser.add_argument(
         '--no-ensure-ascii',

tests/pretty_format_json_test.py

diff --git a/tests/pretty_format_json_test.py b/tests/pretty_format_json_test.py
index 4054b4c..7ce7e16 100644
--- a/tests/pretty_format_json_test.py
+++ b/tests/pretty_format_json_test.py
@@ -3,20 +3,16 @@
 import pytest
 from six import PY2
 
-from pre_commit_hooks.pretty_format_json import parse_indent
+from pre_commit_hooks.pretty_format_json import parse_num_to_int
 from pre_commit_hooks.pretty_format_json import pretty_format_json
 from testing.util import get_resource_path
 
 
-def test_parse_indent():
-    assert parse_indent('0') == ''
-    assert parse_indent('2') == '  '
-    assert parse_indent('\t') == '\t'
-    with pytest.raises(ValueError):
-        parse_indent('a')
-    with pytest.raises(ValueError):
-        parse_indent('-2')
-
+def test_parse_num_to_int():
+    assert parse_num_to_int('0') == 0
+    assert parse_num_to_int('2') == 2
+    assert parse_num_to_int('\t') == '\t'
+    assert parse_num_to_int('  ') == '  '
 
 
 @pytest.mark.parametrize(