add clang-format

.clang-format

+---
+Language: Cpp
+BasedOnStyle: LLVM
+AllowAllParametersOfDeclarationOnNextLine: false
+AlwaysBreakTemplateDeclarations: true
+BinPackArguments: false
+BinPackParameters: false
+BreakConstructorInitializers: BeforeComma
+ColumnLimit: 120
+CommentPragmas: '^(!|NOLINT)'
+ConstructorInitializerAllOnOneLineOrOnePerLine: true
+IncludeBlocks: Regroup
+IncludeCategories: 
+  - Regex: '^<.*'
+    Priority: 2
+  - Regex: '.*'
+    Priority: 1
+IndentCaseLabels: false
+AllowShortCaseLabelsOnASingleLine: true
+IndentWidth: 4
+KeepEmptyLinesAtTheStartOfBlocks: false
+PenaltyReturnTypeOnItsOwnLine: 200
+SpacesBeforeTrailingComments: 1
+TabWidth: 4
+UseTab: Never
+...