Add support for compile_command.json output

[cmaglie]

This implementation is a tentative draft based on @matthijskooijman's #944

I've added a flag --only-compilation-database flag in compile command. When this flag is specified the build run will perform only:

• library discovery
• sketch preprocessing (the preprocessed sketch.ino.cpp is saved in the usual ${build_path}/sketch/... subdir)
• generation of compile_command.json (saved in the build-path). In this case the sketch/core/libraries are not compiled but the command line is saved for compile_command.json

[matthijskooijman]

I finally gotten around to giving this code a whirl, seems to work nicely overall. There's still some points unsolved from #944, but let's further discuss them in #849.

Why did you removing the "Writing compilation database..." message? IMHO it helps to both let people know that this feature is supported in the first place, and helps figure out where the file was written to, especially now it's only written to the build folder. In verbose mode, all commands run are printed, then why not show the compilation database was generated?

[cmaglie]

Why did you removing the "Writing compilation database..." message?

Mainly because:

• it was inside a CompilationDatabase class, from the caller point of view calling SaveToFile should just save or return an error, without any other output
• it used fmt.Printf instead of the logging system

BTW, you have a good point, I'll put a message output on the caller side.