Feature/clang format

If `clang-format` is found the 'clang-format' target is defined. To build this
target, clang format is run on pastis sources (c++) omitting 'packages' sources.

It is better to configure editors to use clang-format than running this on the
whole sources before pushing...

- format is based on LLVM format with a bunch of customizations
- clang-format is deactivated in the `packages` directory