Pulling .clang-format change from @henryiii's 9057962

Author: rwgk

.clang-format

@@ -9,6 +9,7 @@ BinPackParameters: false
 BreakBeforeBinaryOperators: All
 BreakConstructorInitializers: BeforeColon
 ColumnLimit: 99
+IncludeBlocks: Regroup
 IndentCaseLabels: true
 IndentPPDirectives: AfterHash
 IndentWidth: 4
@@ -17,4 +18,19 @@ SpaceAfterCStyleCast: true
 Standard: Cpp11
 StatementMacros: ['PyObject_HEAD']
 TabWidth: 4
+IncludeCategories:
+  - Regex:           '<pybind11/.*'
+    Priority:        -1
+  - Regex:           'pybind11.h"$'
+    Priority:        1
+  - Regex:           '^".*/?detail/'
+    Priority:        1
+    SortPriority:    2
+  - Regex:           '^"'
+    Priority:        1
+    SortPriority:    3
+  - Regex:           '<[[:alnum:]._]+>'
+    Priority:        4
+  - Regex:           '.*'
+    Priority:        5
 ...