Vrapper plugin for clang-format.

This plugin integrates LLVM `clang-format` tool into Vrapper.
Adds three new commands:

- `:clang-format [<style>]` - invokes `clang-format` on the content
  of the current editor. The optional argument specifies the style
  to use (`-style` argument). If omitted the style configured with
  `:clang-format-style` or `:clang-format-default-style` will be
  used. The command accepts line ranges and visual selection range:
  `:.clang-format` will format the current line only. If no range
  is provided the whole file will be reformatted.
- `:clang-format-default-style <style>` - sets the default style to
  be used for all editors.
- `:clang-format-style <style>` - sets the style to be used with the
  current editor only.