+ added .clang-format to .gitignore
The style respects mostly rtp/rtp.{c,h} coding style (original
UltraGrid).
Rather than specifying own style, deduce one from clang-format default
format (just keeping the indention 8 spaces). The style is quite
restrictive because indention of 8 spaces + line with 80 restricts
nesting quite a bit.
Anyways, the style is rather a recommendation and is not enforced in any
way - .clang-format is just added to .gitignore and the developer can
tweak it a bit (like extending ColumnLimit).
