Add format target for source code formatting

[kgugala]

This PR adds make format target which can be used to autoformat the code in the project. The target uses clang-format-5.0 tool.

[mithro]

@kgugala Can you link to the output were the code has been formatted?

Could you also add instructions for @kmurray on how to do the formatting after this pull request lands?

[kgugala]

the results can be found here: https://github.com/antmicro/libblifparse/tree/run-format
to run the format do:

mkdir build && cd build
cmake ..
make format

[vaughnbetz]

@soheilshahrouz : we should probably turn on format checking for this library too. I didn't notice this stray PR until now. Can you take a look and see if we should merge or update?