Make style lookup consistent with clang-format

[arntanguy]

The current version was only

• looking up .clang_format in the file's directory.
• using -style=file option only, causing failure if pwd != file's
  directory

This behavior is quite problematic as it can easily lead to using the wrong .clang-format file, ie the one
in your current pwd instead of the one in the file's project. There are also some obvious cases
where the .clang-format file isn't found.

This commit makes the behavior fully consistent with clang-format:

• lookup .clang_format into the file's directory and recursively go up
  its parent directory until found. If no style file is found, defaults
  to g:clang_format#style_options

I don't really know the Viml language, so please double check if it's written the vim-way.

[arntanguy]

My apologies, I should have looked at the documentation more carefully.
Here is the corrected version.

[arntanguy]

I'm not sure you saw my answer regarding the versions of clang supporting this, so I'll write it here again.

I'm not sure when it --assume-filename got introduced, but clang-3.4 (that you list as dependency in the README) definitely has it.
It seems to be a pretty sensible feature to have, so I would assume it's been there since the first releases of clang-format, but we never know.

Also in clang-format-3.6, there is a -fallback-style that can fallback to any of the preset styles (llvm, google...) if all else fails. This doesn't seem to be in 3.4, so I haven't used it for that pull-request.