Enforce coding style

[DanielGuramulta]

Enforce coding style on commits.

To prevent files mixing different coding styles

// example
if () {
    ...
}

if() {
    ...
}

we make Travis check the style of the files, being edited(patched) in the commit(commit range), and if something is not formatted according to the .clang-format files specifications, we fail the build.

For this to go through, we need:

1. to discuss the coding style and code it into .clang-format
2. format the existing code
3. remove the test commit a3bc159

[dNechita]

This might be annoying as if sorts the included headers alphabetically. You usually want more flexibility on order of the included files.

[DanielGuramulta]

We can also specify include categories, which will provide some general rules on how the included headers will be sorted.

example:
IncludeCategories:
  - Regex:           '^"(llvm|llvm-c|clang|clang-c)/'
    Priority:        2
  - Regex:           '^(<|"(gtest|gmock|isl|json)/)'
    Priority:        3
  - Regex:           '.*'
    Priority:        1

[commodo]

Some commits could use some descriptions

[commodo]

curios: would this work? addons: { apt: { packages: ["clang-format-6.0"], sources: ["llvm-toolchain-${TRAVIS_DIST}-6.0"] } }

also, is ther a generic llvm-toolchain-6.0 addon?
i would not add xenial if possible; otherwise, it's probably better to add dist: xenial back to make sure things stay consistent (sorry about that, if it happens)

[commodo]

commits could use a description