clang-format support for isl

[tobiasgrosser]

It would be nice to automatically format isl patches. The tool clang-format was developed precisely for this: https://clang.llvm.org/docs/ClangFormat.html

Unfortunately, it does not yet seem to support everything needed for isl:

https://bugs.llvm.org/show_bug.cgi?id=33835

[tobiasgrosser]

Travis provides a variable TRAVIS_COMMIT_RANGE which allows us e.g. to only format check the newly added code.