Auto-discover the required clang flags for a project

This uses the clang-flags package, which currently supports detection
of .clang_complete files defining project clang flags. It can be
extended over time to support other methods of determining the flags,
such as compilation databases. Hopefully this means that clang
warnings will be representative without further package configuration
by the user.

Also adds a config option for whether to add the user-configured default
includes to the project-configured ones.
Run clang in driver mode, not frontend mode

Clang shouldn't be run in frontend-only mode directly (http://clang.llvm.org/docs/FAQ.html#id2)
so this switches to running the default driver mode.

This has the benefit of being able to parse exception-based code and
give sensible lint output on it (e.g. boost-based code), which it
couldn't previously.
Tidy code per review comments
Merge pull request #70 from Kev/detect_clang_flags

Detect clang flags
Fix bug in handling of project root directory in erlc
Add description of linter specific configuration to README
Update changelog for 0.13.0
Prepare 0.13.0 release
Extend CONTRIBUTING.md
Dump stacktrace for better debugging
Include the executed command and the env to the result of CommandRunner
Add LinterError for detailed debug output
Dump detailed debug outputs for linter errors
Fix exceeded line length
Update changelog for 0.14.0
Prepare 0.14.0 release
Fix regression of RuboCop execution in 0.14.0
Prepare 0.14.1 release
Use Rake's #sh instead of Kernel#system in tasks
Refactor ::loadGrammar in spec-helper