Allow include paths relative to workspace folder

[letmaik]

I wasn't able to specify an include path relative to the workspace folder. An absolute path worked fine. I'm assuming relative paths currently don't work because the compilation is done from some random temporary folder as current working directory.

Maybe the same syntax as for launch.json could be borrowed:
${workspaceFolder}/my/folder.

[pedro-ricardo]

This will be closed due to be a duplicate of the improvements proposed in #86