Add clang-format config.

I'm going to try it.  I'm not completely happy with the formatting I can
get out of clang-format, but I've been wanting _something_ for so long.

My main remaining gripe is with the use of braces.  I have a simple rule
for those: feel free to leave out the braces on a single-statement block
_so long as matched if/else blocks are formatted identically._  That
does not seem to be something that clang-format can do.

The next-biggest gripe is with alignment and indentation:

    return std::vector<errorhandler *>{std::begin(m_errorhandlers),
                                       std::end(m_errorhandlers)};

That's just horrible.  I don't want that first argument on the same
line as the opening brace — if you've got a list of things that won't
fit the line, start breaking lines right _before_ the first item, so the
structure remains clear at a glance.  Nor do I like creative horizontal
alignment — indent or don't indent, but don't go padding to some
arbitrary width.

It seems that I can turn these horrors off in the configuration, but
only with parentheses, not with braces.  ☹

.clang-format

@@ -0,0 +1,71 @@
+Language: Cpp
+
+AlignAfterOpenBracket: AlwaysBreak
+# AllowAllArgumentsOnNextLine: true
+# AllowAllConstructorInitializersOnNextLine: true
+AllowAllParametersOfDeclarationOnNextLine: true
+AllowShortBlocksOnASingleLine: true
+AllowShortCaseLabelsOnASingleLine: true
+AllowShortFunctionsOnASingleLine: Inline
+# AllowShortIfStatementsOnASingleLine: WithoutElse
+# AllowShortLambdasOnASingleLine: All
+AllowShortLoopsOnASingleLine: true
+AlwaysBreakAfterReturnType: None
+AlwaysBreakBeforeMultilineStrings: true
+# AlwaysBreakTemplateDeclarations: No
+BinPackArguments: true
+BinPackParameters: true
+BreakBeforeBraces: Custom
+BraceWrapping:
+  # AfterCaseLabel: true
+  AfterClass: true
+  AfterControlStatement: true
+  AfterEnum: true
+  AfterExternBlock: true
+  AfterFunction: true
+  AfterNamespace: true
+  AfterStruct: true
+  BeforeCatch: true
+  BeforeElse: true
+  IndentBraces: false
+  SplitEmptyFunction: false
+  SplitEmptyNamespace: false
+  SplitEmptyRecord: false
+BreakBeforeBinaryOperators: None
+BreakBeforeTernaryOperators: false
+BreakConstructorInitializers: AfterColon
+# BreakInheritanceList: AfterColon
+BreakStringLiterals: true
+ColumnLimit: 79
+ConstructorInitializerAllOnOneLineOrOnePerLine: true
+ConstructorInitializerIndentWidth: 8
+ContinuationIndentWidth: 2
+Cpp11BracedListStyle: true
+FixNamespaceComments: true
+IncludeBlocks: Preserve
+IndentCaseLabels: false
+IndentPPDirectives: AfterHash
+IndentWidth: 2
+IndentWrappedFunctionNames: false
+KeepEmptyLinesAtTheStartOfBlocks: false
+MaxEmptyLinesToKeep: 2
+# NamespaceIndentation: All
+SortIncludes: true
+SortUsingDeclarations: true
+SpaceAfterCStyleCast: false
+SpaceAfterTemplateKeyword: false
+SpaceBeforeAssignmentOperators: true
+# SpaceBeforeCpp11BracedList: false
+# SpaceBeforeCtorInitializerColon: true
+# SpaceBeforeInheritanceColon: true
+# SpaceBeforeParents: ControlStatements
+# SpaceBeforeRangedBasedForLoopColon: true
+SpaceInEmptyParentheses: false
+SpacesInAngles: false
+SpacesInCStyleCastParentheses: false
+SpacesInContainerLiterals: false
+SpacesInParentheses: false
+SpacesInSquareBrackets: false
+Standard: Cpp11
+UseTab: Never
+---