Clang-format configuration added.

Some notes on the configuration options chosen:

* We want `AllowShortEnumsOnASingleLine` set to `false`, but that
option is clang-format-11 and up, which is not out yet.
* `IndentPPDirectives` is currently inconsistent, but `AfterHash`
is the preferred style in new code.
* `NamespaceIndentation` is a mess, but `All` is closer to the effect
we want than `Inner`.
* `SpacesInParentheses` set to `true` is not ideal due to it also
introducing extra spaces in preprocessor expressions, but using it
is much closer to the current style than not.

All in all, using this setting globally would reformat pretty much
every line of code in the codebase, but it is as close as possible
to the bespoke style currently used. Still, it should only be used
on the diffs.

Closes catchorg#1182

.clang-format

@@ -0,0 +1,25 @@
+---
+AccessModifierOffset: '-4'
+AlignEscapedNewlines: Left
+AllowAllConstructorInitializersOnNextLine: 'true'
+BinPackArguments: 'false'
+BinPackParameters: 'false'
+BreakConstructorInitializers: AfterColon
+ConstructorInitializerAllOnOneLineOrOnePerLine: 'true'
+DerivePointerAlignment: 'false'
+FixNamespaceComments: 'true'
+IncludeBlocks: Regroup
+IndentCaseLabels: 'false'
+IndentPPDirectives: AfterHash
+IndentWidth: '4'
+Language: Cpp
+NamespaceIndentation: All
+PointerAlignment: Left
+SpaceBeforeCtorInitializerColon: 'false'
+SpaceInEmptyParentheses: 'false'
+SpacesInParentheses: 'true'
+Standard: Cpp11
+TabWidth: '4'
+UseTab: Never
+
+...