add output_dir feature

[ilev4ik]

This pull request is about to provide a support of writing out compile_commands.json relatively to workspace root to provide each project means of customizing their .clangd.

By default compile-commands-extractor's refresh_compile_command produces its output into bazel-workspace root. But our monorepository is single-rooted workspace and has many subprojects as children folders. Multiple projects are maintained by one team of developers, but a single project may have its own clangd configuration.

I didn't find any customization attribute of bazel target so created this PR. If there exist any please let me know. TY

@cpsauer it would be great if you find some time to review PR :)

Closes #217